Implements the topological mosaic pipeline from docs/design/mosaic.md, turning
`--hierarchical cutout` into a true gapless tessellation instead of the old
re-cluster-and-retrace fake.
LabelMap (flatten Segmentation top-down)
→ boundary-graph extraction (integer-exact: corners, node rule, segment
and ring tracing on the pixel-corner lattice)
→ face assembly (left-region successor rule; winding falls out,
so each region is one nonzero-fill path)
→ fit each segment ONCE (shared by both adjacent faces, reversed
exactly → byte-identical shared boundaries)
→ compose per-region paths
Backends: PixelSegmentFitter (exact reference) and PolygonSegmentFitter
(symmetric open Douglas-Peucker collapsing staircases to the crack midline).
The spline segment fitter is still pending; mosaic + spline currently falls
back to polygon.
Compositing now owns its fitter (Stacked(CurveFitter) / Mosaic(SegmentFitter)).
Tests: single region, vertical split, T-junction, checkerboard pinch, nested
rings, border-touching, and a pixel round-trip property test over 40 random
maps (rasterize composed faces == input label map). Plus two mosaic goldens.
12 synthetic-image cases covering every stage: all three fitters, holes,
region adjacency, hierarchical layering, binary mode, fixed-palette and
auto-quantize color fitting, and the three optimizer/writer levels.
Fixtures are built from in-code images, not the JPEG samples, because JPEG
decoding is image-crate-version dependent and would make goldens fragile.
Regenerate after an intentional change with VTRACER_BLESS=1.
Chris Tsang