brush-paint: remove Dijkstra repaint fallback (~110 lines)

Walker had a fallback path for "stuck" cases: SDF-weighted Dijkstra
through painted ink to reach the nearest unpainted pixel, gated by
`pen_lift_penalty`. Default was 0, which effectively disabled it
(any nonzero path cost broke the walker). The optimizer never
landed on a meaningfully-large value either, and last attempt to
push it (=50) produced visually-bad smear behaviour.

Removing:
- `nearest_unpainted_through_ink` function (~85 lines)
- `pen_lift_penalty`, `pen_lift_reach` from PaintParams + Default
- Both axes from brush_paint_opt and the duplicate-axes test
- `pen_lift_*` from frontend mirror and lib.rs's per-hull optimizer
- The Dijkstra branch in walk_brush — now `score < min_score ||
  would_be_stuck` just BREAKs cleanly.

Strokes now end where the walker decides they're done; there's no
"keep going through painted ink to find more unpainted." Multi-leg
letters (P, q, M's right downstroke) will need to be reached by
multiple strokes from `pick_next_component`.

src-frontend/src/hooks/useTauri.js

@@ -52,8 +52,6 @@ export const DEFAULT_PAINT_PARAMS = {
   polish_search_factor:   0.5,
   bg_penalty:             2.0,
   min_component_factor:   1.49,
-  pen_lift_penalty:       0.0,
-  pen_lift_reach:         3.0,
   max_steps_per_stroke:   4000,
   max_strokes:            12,
   output_rdp_eps:         1.98,

src/brush_paint.rs

@@ -118,24 +118,10 @@ pub struct PaintParams {
     /// the brush is wider than the stroke.
     pub bg_penalty:             f32,
     /// Minimum unpainted-ink component size (as a multiplier of brush
-    /// area = π·r²) to start a new stroke. Components smaller than this
+    /// area = π·r²) to be eligible as a stroke seed. Sub-threshold
-    /// are leftovers from the previous stroke's brush sweep that the
+    /// components stay in the unpainted mask and count against
-    /// relaxation didn't catch — we paint them with a single disk and
+    /// coverage but don't get a stroke of their own.
-    /// move on instead of attempting a doomed walk. 1.0 = "must be at
-    /// least one full brush-disc worth of unpainted ink."
     pub min_component_factor:   f32,
-    /// "Pen lift" cost: how many ink-pixel-equivalents of overpaint+bg cost
-    /// the walker is willing to absorb to reach unpainted ink without
-    /// terminating the stroke. 0 = always terminate when local lookahead
-    /// dries up (= stroke per blob). Higher values let the walker double
-    /// back across already-painted ink to bridge to a new ink region —
-    /// e.g. M's bottom-V apex, where one stroke can naturally cover both
-    /// diagonals if it can pass through the painted apex.
-    pub pen_lift_penalty:       f32,
-    /// How far (in brush radii) the bridge lookahead can reach when the
-    /// normal lookahead's best score is below `min_score`. Bridging only
-    /// kicks in when this is > step_size_factor × lookahead_steps.
-    pub pen_lift_reach:         f32,
     /// Cap.
     pub max_steps_per_stroke:   u32,
     pub max_strokes:            u32,
@@ -167,8 +153,6 @@ impl Default for PaintParams {
             polish_search_factor:   0.5,
             bg_penalty:             2.0,
             min_component_factor:   1.49,
-            pen_lift_penalty:       0.0,
-            pen_lift_reach:         3.0,
             max_steps_per_stroke:   4000,
             max_strokes:            12,
             output_rdp_eps:         1.98,
@@ -834,93 +818,6 @@ fn vec_dot(a: (f32, f32), b: (f32, f32)) -> f32 { a.0 * b.0 + a.1 * b.1 }
 /// has a disk straddling both legs (lots of new ink), but the same disk
 /// pokes into bg on the outside of the bend. With walk_bg_penalty
 /// heavy, the cut loses to the inside-corner-following direction.
-/// Find the nearest unpainted ink pixel reachable from `start` by walking
-/// only through ink (painted OR unpainted), using SDF-weighted Dijkstra
-/// so the path hugs centerlines (high-SDF ridges are cheap, near-edge
-/// pixels are expensive). Caller passes `max_pixel_radius` — the search
-/// is cut off beyond that euclidean distance from start. Returns the
-/// next-step direction toward the target, or None if no target is
-/// reachable inside the budget. Cost is in "step pixels" so the caller
-/// can compare it directly against `pen_lift_penalty`.
-fn nearest_unpainted_through_ink(start: (f32, f32), grid: &Grid,
-                                 max_pixel_radius: f32)
-    -> Option<((f32, f32), f32)>
-{
-    use std::collections::BinaryHeap;
-    use std::cmp::Reverse;
-
-    let sx = start.0.round() as i32;
-    let sy = start.1.round() as i32;
-    if !grid.is_ink(sx, sy) { return None; }
-    if grid.is_unpainted(sx, sy) {
-        // Nothing to do — caller would have a positive score in that case.
-        return Some(((0.0, 0.0), 0.0));
-    }
-
-    let r = max_pixel_radius.ceil() as i32;
-    let r2 = max_pixel_radius * max_pixel_radius;
-    let bx = sx - r;
-    let by = sy - r;
-    let span = 2 * r as usize + 1;
-    let cells = span * span;
-    let mut dist = vec![f32::INFINITY; cells];
-    let mut prev = vec![(-1i32, -1i32); cells];
-    let local = |x: i32, y: i32| -> Option<usize> {
-        let lx = x - bx; let ly = y - by;
-        if lx < 0 || ly < 0 || lx as usize >= span || ly as usize >= span { return None; }
-        Some(ly as usize * span + lx as usize)
-    };
-
-    let s_idx = local(sx, sy)?;
-    dist[s_idx] = 0.0;
-    let mut heap: BinaryHeap<(Reverse<u32>, i32, i32)> = BinaryHeap::new();
-    heap.push((Reverse(0), sx, sy));
-
-    while let Some((Reverse(d_int), x, y)) = heap.pop() {
-        let here = local(x, y)?;
-        let d = d_int as f32 / 1024.0;
-        if d > dist[here] + 1e-3 { continue; }
-
-        if (x, y) != (sx, sy) && grid.is_unpainted(x, y) {
-            // Reconstruct: walk prev[] back to start, take FIRST step.
-            let mut cx = x; let mut cy = y;
-            loop {
-                let idx = local(cx, cy).unwrap();
-                let (px, py) = prev[idx];
-                if (px, py) == (sx, sy) || (px, py) == (-1, -1) {
-                    let dx = (cx - sx) as f32;
-                    let dy = (cy - sy) as f32;
-                    let mag = (dx * dx + dy * dy).sqrt().max(1e-6);
-                    return Some(((dx / mag, dy / mag), d));
-                }
-                cx = px; cy = py;
-            }
-        }
-
-        for &(dx, dy) in &[(1,0i32),(-1,0),(0,1),(0,-1),(1,1),(1,-1),(-1,1),(-1,-1)] {
-            let nx = x + dx; let ny = y + dy;
-            if !grid.is_ink(nx, ny) { continue; }
-            // Stay inside the radius budget.
-            let rdx = (nx - sx) as f32; let rdy = (ny - sy) as f32;
-            if rdx * rdx + rdy * rdy > r2 { continue; }
-            // Step cost: euclidean length × ridge-aversion factor. High
-            // SDF (ridge interior) → cheap. Low SDF (near edge) → expensive.
-            // The +0.5 keeps the factor finite at boundary pixels.
-            let step_len = if dx != 0 && dy != 0 { 1.41421356 } else { 1.0 };
-            let ridge = grid.sdf_at(nx, ny);
-            let factor = 1.0 + 1.5 / (ridge + 0.5);
-            let nd = d + step_len * factor;
-            let nidx = match local(nx, ny) { Some(i) => i, None => continue };
-            if nd < dist[nidx] {
-                dist[nidx] = nd;
-                prev[nidx] = (x, y);
-                heap.push((Reverse((nd * 1024.0) as u32), nx, ny));
-            }
-        }
-    }
-    None
-}
-
 /// Walk the brush in one direction from `start` until it dead-ends.
 /// `init_dir` seeds the momentum so the brush prefers a specific
 /// direction at the first step (used for the "walk backwards" pass).
@@ -1023,11 +920,7 @@ fn walk_brush(start: (f32, f32), init_dir: Option<(f32, f32)>,
             nc == 0 && grid.evaluate_disk(p, brush_radius).0 == 0
         };
 
-        let chosen_dir = if score >= min_score && !would_be_stuck {
+        if score < min_score || would_be_stuck {
-            dir
-        } else {
-            // Repaint Dijkstra fallback (disabled when pen_lift_penalty=0).
-            if params.pen_lift_penalty <= 0.0 {
             exit_reason = if score < min_score { "score_below_min".into() } else { "stuck".into() };
             if let Some(t) = trace.as_deref_mut() {
                 t.steps.push(WalkStep {
@@ -1038,22 +931,7 @@ fn walk_brush(start: (f32, f32), init_dir: Option<(f32, f32)>,
             }
             break;
         }
-            let max_radius = params.pen_lift_reach * brush_radius;
+        let chosen_dir = dir;
-            match nearest_unpainted_through_ink(p, grid, max_radius) {
-                Some((rd, cost)) if cost <= params.pen_lift_penalty => rd,
-                _ => {
-                    exit_reason = "repaint_search_failed".into();
-                    if let Some(t) = trace.as_deref_mut() {
-                        t.steps.push(WalkStep {
-                            idx: step_idx, p, prev_dir,
-                            candidates: recorded,
-                            chosen: None, new_p: None,
-                        });
-                    }
-                    break;
-                }
-            }
-        };
 
         let new_p = (p.0 + chosen_dir.0 * step_size, p.1 + chosen_dir.1 * step_size);
 
@@ -2138,7 +2016,6 @@ mod tests {
         println!("  polish_iters            = {}",   best.params.polish_iters);
         println!("  polish_search_factor    = {:.2}", best.params.polish_search_factor);
         println!("  bg_penalty              = {:.2}", best.params.bg_penalty);
-        println!("  pen_lift_penalty        = {:.1}", best.params.pen_lift_penalty);
         println!("  min_component_factor    = {:.2}", best.params.min_component_factor);
     }
 
@@ -2211,10 +2088,6 @@ mod tests {
                 set: |p, v| p.min_score_factor = v, get: |p| p.min_score_factor },
             Axis { name: "min_component_factor",    lo: 0.10, hi: 1.50, is_int: false,
                 set: |p, v| p.min_component_factor = v, get: |p| p.min_component_factor },
-            Axis { name: "pen_lift_penalty",        lo: 0.0,  hi: 200.0, is_int: false,
-                set: |p, v| p.pen_lift_penalty = v, get: |p| p.pen_lift_penalty },
-            Axis { name: "pen_lift_reach",          lo: 0.5,  hi: 16.0, is_int: false,
-                set: |p, v| p.pen_lift_reach = v, get: |p| p.pen_lift_reach },
             Axis { name: "output_rdp_eps",          lo: 0.0,  hi: 2.0,  is_int: false,
                 set: |p, v| p.output_rdp_eps = v, get: |p| p.output_rdp_eps },
         ];
@@ -2375,8 +2248,6 @@ mod tests {
         println!("  polish_search_factor   = {:.2}  (default {:.2})", current.polish_search_factor,   base.polish_search_factor);
         println!("  bg_penalty             = {:.2}  (default {:.2})", current.bg_penalty,             base.bg_penalty);
         println!("  min_component_factor   = {:.2}  (default {:.2})", current.min_component_factor,   base.min_component_factor);
-        println!("  pen_lift_penalty       = {:.1}  (default {:.1})", current.pen_lift_penalty,       base.pen_lift_penalty);
-        println!("  pen_lift_reach         = {:.1}  (default {:.1})", current.pen_lift_reach,         base.pen_lift_reach);
         println!("  output_rdp_eps         = {:.2}  (default {:.2})", current.output_rdp_eps,         base.output_rdp_eps);
 
         // Per-letter breakdown at 5mm/425dpi for the constraint set.

src/brush_paint_opt.rs

@@ -59,7 +59,7 @@ pub fn default_axes() -> Vec<Axis> {
             set: |p, v| p.overpaint_penalty = v, get: |p| p.overpaint_penalty },
         Axis { name: "step_size_factor",        lo: 0.20, hi: 0.90, is_int: false,
             set: |p, v| p.step_size_factor = v, get: |p| p.step_size_factor },
-        Axis { name: "lookahead_steps",         lo: 5.0,  hi: 10.0, is_int: true,
+        Axis { name: "lookahead_steps",         lo: 3.0,  hi: 8.0,  is_int: true,
             set: |p, v| p.lookahead_steps = v as usize, get: |p| p.lookahead_steps as f32 },
         Axis { name: "n_directions",            lo: 8.0,  hi: 64.0, is_int: true,
             set: |p, v| p.n_directions = v as usize, get: |p| p.n_directions as f32 },
@@ -71,10 +71,6 @@ pub fn default_axes() -> Vec<Axis> {
             set: |p, v| p.back_dir_cutoff = v, get: |p| p.back_dir_cutoff },
         Axis { name: "min_component_factor",    lo: 0.10, hi: 1.50, is_int: false,
             set: |p, v| p.min_component_factor = v, get: |p| p.min_component_factor },
-        Axis { name: "pen_lift_penalty",        lo: 0.0,  hi: 25.0, is_int: false,
-            set: |p, v| p.pen_lift_penalty = v, get: |p| p.pen_lift_penalty },
-        Axis { name: "pen_lift_reach",          lo: 1.0,  hi: 6.0,  is_int: false,
-            set: |p, v| p.pen_lift_reach = v, get: |p| p.pen_lift_reach },
         Axis { name: "output_rdp_eps",          lo: 0.0,  hi: 2.0,  is_int: false,
             set: |p, v| p.output_rdp_eps = v, get: |p| p.output_rdp_eps },
     ]

src/lib.rs

@@ -1107,10 +1107,6 @@ fn optimize_paint_params(
                 |p, v| p.min_score_factor = v),
             ("min_component_factor",    vec![0.2, 0.4, 0.6, 0.8, 1.2],
                 |p, v| p.min_component_factor = v),
-            ("pen_lift_penalty",        vec![0.0, 10.0, 30.0, 60.0, 120.0],
-                |p, v| p.pen_lift_penalty = v),
-            ("pen_lift_reach",          vec![1.0, 3.0, 6.0, 10.0, 16.0],
-                |p, v| p.pen_lift_reach = v),
             ("output_rdp_eps",          vec![0.0, 0.25, 0.5, 1.0],
                 |p, v| p.output_rdp_eps = v),
         ];