Derive the watershed cutout merge tolerance from the detail dial

The detail dial has no color units - it targets a region count
(2^(detail/25.5)) and the cut threshold is volume persistence - so a
cutout merge tolerance cannot fall out of it dimensionally. Anchor it
instead: at max detail the user asked for every distinction the hierarchy
can make (merge only identical colors, as before), and at the default
detail (128) it matches the color-cluster default gradient step (16),
which is the tolerance the cutout merge was designed around. Linear in
between: merge_diff = (255 - detail) / 8, reaching 1 at detail 247.

Floor the watershed cutout merge tolerance at a just-noticeable difference

Faces a human cannot tell apart (e.g. #863339 next to #863238, 2 L1
apart) are pointless as separate patches at any detail, so the derived
tolerance becomes max(2, (255 - detail) / 8): the 248..=255 band merges
sub-JND neighbours instead of nothing. The default-detail anchor (16, the
color-cluster default gradient step) is unchanged. Cityscape cutout at
max detail: 992 faces down to 886.

crates/vtracer/src/config.rs

@@ -293,14 +293,18 @@ impl Config {
             Hierarchical::Stacked => Compositing::Stacked(self.fitter()),
             Hierarchical::Cutout => Compositing::Mosaic {
                 fitter: self.segment_fitter(),
-                // Rejoin flattened neighbours the gradient layering split:
-                // clustering itself considers colors within one gradient step
-                // to be the same region (`deepen_diff`). The watershed
-                // hierarchy already decided every merge, so its partition
-                // keeps its shape — only identical-color neighbours (e.g.
-                // after a palette snap) still collapse into one face.
+                // Rejoin flattened neighbours the clustering split too finely.
+                // Color clustering considers colors within one gradient step
+                // to be the same region (`deepen_diff`), so that is its
+                // tolerance. The watershed dial has no color units (it
+                // targets a region *count*), so its tolerance is anchored
+                // instead: at the default detail (128) it matches the
+                // color-cluster default gradient step (16) and grows linearly
+                // as detail drops; the floor keeps faces a human cannot tell
+                // apart (within a just-noticeable difference) from surviving
+                // as separate patches even at maximum detail.
                 merge_diff: match self.clustering {
-                    Clustering::Watershed => 0,
+                    Clustering::Watershed => ((255 - self.watershed_detail as i32) / 8).max(2),
                     _ => self.layer_difference,
                 },
             },

crates/vtracer/tests/watershed.rs

@@ -244,13 +244,16 @@ fn session_recut_matches_one_shot() {
     }
 }
 
-/// Watershed + cutout is native: the partition reaches the mosaic untouched,
-/// so two regions within one gradient step stay separate faces (the color
-/// path's `merge_similar` would have rejoined them).
+/// Watershed + cutout is native: at max detail the partition reaches the
+/// mosaic essentially untouched, so two *distinguishable* regions within one
+/// gradient step stay separate faces (the color path's `merge_similar` would
+/// have rejoined them). Only the just-noticeable-difference floor applies —
+/// see `cutout_merge_tolerance_follows_detail`.
 #[test]
 fn cutout_keeps_watershed_partition() {
-    // Two halves 4 gray-levels apart: close enough that the flatten merge
-    // (threshold = layer_difference = 16 >= 3*4) would union them.
+    // Two halves 4 gray-levels apart (12 L1): close enough that the flatten
+    // merge (threshold = layer_difference = 16 >= 3*4) would union them, yet
+    // clearly above the JND floor (2).
     let img = image(32, 20, |x, _| {
         if x < 16 {
             (100, 100, 100)
@@ -273,6 +276,45 @@ fn cutout_keeps_watershed_partition() {
     );
 }
 
+/// The cutout merge tolerance is derived from the detail dial —
+/// `max(2, (255 − detail) / 8)` — because detail has no color units of its
+/// own. The same two halves 12 L1 apart that max detail keeps separate (see
+/// above) merge into one face at the default detail, whose tolerance (15)
+/// matches the color-cluster default gradient step; and a pair a human
+/// cannot tell apart (within the just-noticeable-difference floor) merges
+/// even at max detail.
+#[test]
+fn cutout_merge_tolerance_follows_detail() {
+    let halves = |a: (u8, u8, u8), b: (u8, u8, u8)| {
+        image(32, 20, |x, _| if x < 16 { a } else { b })
+    };
+    let cfg = |detail| Config {
+        clustering: Clustering::Watershed,
+        hierarchical: Hierarchical::Cutout,
+        watershed_detail: detail,
+        filter_speckle: 0,
+        ..Config::default()
+    };
+
+    let img = halves((100, 100, 100), (104, 104, 104));
+    let doc = cfg(128).build().unwrap().run(&img).unwrap();
+    assert_eq!(
+        doc.shapes.len(),
+        1,
+        "near-identical neighbours merge at the default detail"
+    );
+
+    // #863339 next to #863238 (2 L1 apart): indistinguishable by eye, so it
+    // must never survive as two patches, not even at maximum detail.
+    let img = halves((0x86, 0x33, 0x39), (0x86, 0x32, 0x38));
+    let doc = cfg(255).build().unwrap().run(&img).unwrap();
+    assert_eq!(
+        doc.shapes.len(),
+        1,
+        "sub-JND neighbours merge even at max detail"
+    );
+}
+
 /// Regions are 4-connected: two same-colored squares touching only at a
 /// corner are separate basins (and so are the two squares of the other color).
 #[test]