Add mosaic spline fitter; fix stacked holes & relative writer; add test suite

Feature — mosaic spline segment fitter (crates/vtracer/src/mosaic/fit.rs):
open-path cubic fitting for boundary segments, reusing the now-public
visioncortex primitives (PathSimplify::limit_penalties for symmetric,
gap-free staircase removal; open-path SubdivideSmooth::{find_corners,
subdivide_keep_corners,find_splice_points}; fit_points_with_bezier per splice
slice). Matches stacked spline curve quality; endpoints pinned to lattice
nodes so shared boundaries stay seam-free.

Fix — stacked mode punched holes in cluster masks (to_image_with_hole .. true);
stacked must trace solid layers and occlude by paint-order overdraw (false).
Holes left the layer below exposed as hairline seams.

Fix — the relative SVG writer measured a subpath's opening `m` from the last
vertex instead of the subpath start (SVG resets the current point to the start
after Z), misplacing holes / extra subpaths at optimize=1/2.

Tests — new tests/equivalence.rs: stacked-vs-mosaic interior agreement (all
fitters) and a seam guard (a full-coverage image must render fully opaque).
svg round-trip test (absolute vs relative encode identical geometry). mosaic
spline endpoint-pinning test. Regenerated goldens; added disc_mosaic_spline.
resvg added as a dev-dependency (test-only; not compiled for wasm).

Drop unused MosaicOptions placeholder

The strict/seam-stroke mitigations aren't needed — the mosaic geometry is
already gapless and seam-free. Remove the no-op MosaicOptions struct and thread
it out of Compositing::Mosaic and compose_mosaic.

crates/vtracer/Cargo.toml

@@ -16,3 +16,8 @@ path = "src/lib.rs"
 
 [dependencies]
 visioncortex.workspace = true
+
+[dev-dependencies]
+# Rasterize-and-diff equivalence tests (stacked vs mosaic). Test-only; not
+# compiled for wasm targets, so the library stays wasm-safe.
+resvg = "0.45"