Fix spline ballooning on sparse slices (visioncortex 0.9.1)

A splice slice with very uneven point spacing — a few-pixel jog followed
by a long straight leg, which the walker produces around thin strands —
was fitted by a single cubic that interpolated every sample exactly while
swinging up to ~30 px sideways between them (the fit error was only
measured at the samples). On the Cityscape sample at -p 8 -g 28 the
maroon strand's wall bulged across its 2 px gap; a long-standing defect
present in pre-1.0 vtracer as well.

The fix lives in visioncortex 0.9.1 (densify slices with witness points,
keep the full multi-cubic chain): the mosaic's shared-boundary spline
fitter switches to the new fit_points_with_beziers so open segments get
the same treatment as stacked mode. Verified against the exact pixel-mode
rasterization — the fitted wall now matches it at every probed row — with
identical path counts and only +3 cubics across the whole Cityscape
output. All three workspaces (core, py, nodejs) carry a [patch.crates-io]
entry pointing at the local visioncortex until 0.9.1 is published.

Regression coverage for spline handle anchoring; re-bless goldens

tests/spline_fit.rs traces the real reproduction (the Cityscape sample at
color precision 8 / gradient step 28, whose maroon region contains a 1 px
by 330 px strand) through both compositors and asserts the property the
sparse-slice bug violated: every cubic's control points stay within its
shape's on-curve bounding box plus a 15 px allowance. The ballooning fit
put a handle 25 px outside the whole shape, so the margin is decisive in
both directions. The sample photo is decoded via a test-only image
dev-dependency.

The synthetic goldens are re-blessed: the fitting fix shifts fixture
coordinates slightly (witness points refine fits even on clean shapes);
the old files still passed the render-diff, so the visual change is
sub-tolerance, but goldens should byte-match intentional behavior.

Depend on published visioncortex 0.9.1; drop local path patches

CHANGELOG.md

@@ -18,6 +18,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 * `color_mode` is replaced by `clustering` (`color-cluster` | `bw` | `watershed`) across the CLI (`--clustering`), Rust (`Config::clustering`, enum `Clustering`), Python, and Node — the field selects the region-forming algorithm, not a color space.
 
+### Fixed
+
+* Spline fitting no longer swings far away from the outline around thin strands (a long-standing defect, via visioncortex 0.9.1): a sparse splice slice — a few-pixel jog followed by a long straight leg — could be fitted by a single cubic that interpolated every sample while ballooning up to ~30 px sideways between them, visibly crossing narrow gaps. Slices are now densified before fitting and multi-cubic fits are kept in full, in both stacked mode and the mosaic's shared-boundary fitter.
+
 ## 1.0.0-alpha.1 - 2026-07-24
 
 Ground-up rewrite of VTracer into a **vectorization framework** with pluggable stages.