Cutout: merge neighbouring mosaic regions within one gradient step

The stacked hierarchy deliberately splits smooth areas into gradient layers
one deepen_diff apart — that is what makes stacking look continuous. When
cutout flattens those layers into a mosaic, the layering degenerates into
abutting faces with barely distinguishable fills that clustering would have
treated as one region.

Add LabelMap::merge_similar: agglomerative union-find over the flattened
adjacency graph using the clustering color metric (sum of per-channel
absolute diffs, merge when <= deepen_diff). Most-similar pairs union first
and each merged region's color is re-derived as the area-weighted mean, so
gradient chains only coalesce while they genuinely stay within the
threshold — no transitive collapse. compose_mosaic runs it between
flattening and boundary extraction; Compositing::Mosaic carries the
threshold and Config wires it to layer_difference (gradient step), so there
is no new knob.

On the gum-tree sample (poster preset, cutout) this drops 919 faces to 745
with no visible difference. Covered by unit tests for the merge semantics
(running means, OUTSIDE handling, zero threshold) plus a compose-level test
that gradient strips coalesce into one face; goldens and the stacked/cutout
equivalence suite are unaffected.

CHANGELOG.md

@@ -9,9 +9,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Added
 
-* Progress reporting and cancellation for driving a UI from a worker thread: `Pipeline::run_with_progress` with a `CancelToken` and a per-phase callback.
-* `Session`: interactive tuning that clusters an image once and re-renders per `Config` change, re-clustering only when a clustering parameter actually changes. Built on `Pipeline::segment`/`finish` (cache the segmentation, re-run just color/curve/optimize); `Config::segment_key` exposes what it compares.
 * Binary thresholding: a tunable fixed threshold and Bradley–Roth adaptive thresholding for uneven lighting — CLI `--threshold` / `--adaptive` (`--adaptive-window`, `--adaptive-t`), also on `Config`, Python, and Node.
+* Cutout mode merges neighbouring mosaic regions whose colors are within one gradient step — the flattened tessellation no longer keeps the near-identical faces that stacked gradient layering splits a smooth area into.
 
 ## 1.0.0-alpha.1 - 2026-07-24