fix: weight slider, Output node preview, blend_mode serialization

Three bugs fixed:

1. Weight slider had no effect — evaluate_graph called apply_layer but never
   applied layer.weight. Now applies the same lerp as the old apply_stack:
   effective = clamp(255*(1-w) + raw*w, 0, 255). w=0 → background, w=1 →
   identity, w>1 → amplified toward ink.

2. Output node showed thresholded binary (vizB64) instead of the raw
   response map. Root cause: Output was filtered out of node_previews, so
   NodeGraph fell back to outputImageB64 which is the post-threshold viz.
   Fix: include Output in node_previews (only Source is excluded); NodeGraph
   now uses nodePreviews[node.id] for all nodes except Source. The threshold
   slider in Hulls & Contours no longer affects detection graph thumbnails.

3. Combine blend_mode was always "Average" because the frontend stored the
   field as blendMode (camelCase) but the Rust payload expected blend_mode
   (snake_case) — serde found nothing and defaulted. Changed to snake_case
   throughout NodeGraph.jsx to match the rest of the payload convention.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src-frontend/src/App.jsx

@@ -417,7 +417,6 @@ export default function App() {
               }}
               nodePreviews={passes[activePass].nodePreviews}
               sourceImageB64={image?.preview_b64 ?? null}
-              outputImageB64={passes[activePass]?.vizB64 ?? null}
             />
           ) : (
             <Viewport

src-frontend/src/components/NodeGraph.jsx

@@ -39,7 +39,7 @@ function bezier(from, to) {
 }
 
 // ── Component ──────────────────────────────────────────────────────────────────
-export default function NodeGraph({ graph, onChange, nodePreviews, sourceImageB64, outputImageB64 }) {
+export default function NodeGraph({ graph, onChange, nodePreviews, sourceImageB64 }) {
   const canvasRef = useRef(null)
   const [pan,  setPan]  = useState({ x: 40, y: 40 })
   const [zoom, setZoom] = useState(1)
@@ -190,7 +190,7 @@ export default function NodeGraph({ graph, onChange, nodePreviews, sourceImageB6
     const id = newNodeId(kind)
     const node = kind === 'Kernel'
       ? { id, kind, x, y, ...defaultKernelProps() }
-      : { id, kind, x, y, blendMode: 'Average', inputCount: 2 }
+      : { id, kind, x, y, blend_mode: 'Average', inputCount: 2 }
     const g = graphRef.current
     onChangeRef.current({ ...g, nodes: [...g.nodes, node] })
   }
@@ -226,10 +226,9 @@ export default function NodeGraph({ graph, onChange, nodePreviews, sourceImageB6
                    : (node.kind === 'Kernel' || node.kind === 'Output') ? 1 : 0
     const hasOut   = node.kind !== 'Output'
 
-    // Preview image: explicit source/output images, or per-node detection map
-    const preview = node.kind === 'Source'  ? sourceImageB64
-                  : node.kind === 'Output'  ? outputImageB64
-                  : nodePreviews?.[node.id]
+    // Source shows the original image; all other nodes (including Output)
+    // show the raw response map from nodePreviews — consistent and threshold-independent.
+    const preview = node.kind === 'Source' ? sourceImageB64 : nodePreviews?.[node.id]
 
     const accentColor = node.kind === 'Source'  ? '#7c3aed'
                       : node.kind === 'Output'  ? '#b45309'
@@ -326,11 +325,11 @@ export default function NodeGraph({ graph, onChange, nodePreviews, sourceImageB6
             {node.kind === 'Combine' && (<>
               <div style={{ display: 'flex', flexWrap: 'wrap', gap: 2 }}>
                 {BLEND_MODES.map(m => (
-                  <button key={m} onMouseDown={e => e.stopPropagation()} onClick={() => updateNode(node.id, { blendMode: m })}
+                  <button key={m} onMouseDown={e => e.stopPropagation()} onClick={() => updateNode(node.id, { blend_mode: m })}
                     style={{
                       padding: '1px 5px', borderRadius: 3, fontSize: 10, cursor: 'pointer', border: 'none',
-                      background: node.blendMode === m ? '#0f766e' : '#1e293b',
-                      color: node.blendMode === m ? '#fff' : '#94a3b8',
+                      background: node.blend_mode === m ? '#0f766e' : '#1e293b',
+                      color: node.blend_mode === m ? '#fff' : '#94a3b8',
                     }}
                   >{m}</button>
                 ))}

src/detect.rs

@@ -505,7 +505,18 @@ pub fn evaluate_graph(
         let node = node_map[id];
         let result: Option<Vec<u8>> = match &node.kind {
             NodeKind::Source => None,
-            NodeKind::Kernel(layer) => Some(apply_layer(rgb, layer)),
+            NodeKind::Kernel(layer) => {
+                let raw = apply_layer(rgb, layer);
+                let w = layer.weight;
+                // w=0 → full background, w=1 → identity, w>1 → amplify toward ink
+                Some(if (w - 1.0).abs() < 1e-6 {
+                    raw
+                } else {
+                    raw.iter().map(|&r| {
+                        (255.0 * (1.0 - w) + r as f32 * w).clamp(0.0, 255.0) as u8
+                    }).collect()
+                })
+            }
             NodeKind::Combine(mode) => {
                 let mut ins = incoming[id].clone();
                 ins.sort_by_key(|&(_, p)| p);
@@ -530,12 +541,12 @@ pub fn evaluate_graph(
         .and_then(|n| outputs.get(n.id.as_str()).cloned())
         .unwrap_or_else(bg);
 
-    // Per-node previews — omit Source (no output) and Output (same as final_map)
+    // Per-node previews — omit only Source (no map output); include Output so
+    // the graph editor can show the raw response map (not the thresholded vizB64).
     let previews: HashMap<String, Vec<u8>> = outputs.into_iter()
         .filter(|(id, _)| {
             let key: &str = id;
-            node_map.get(key).map_or(true, |n|
-                !matches!(n.kind, NodeKind::Source | NodeKind::Output))
+            node_map.get(key).map_or(true, |n| !matches!(n.kind, NodeKind::Source))
         })
         .map(|(id, map)| (id.to_string(), map))
         .collect();