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

crates/vtracer/Cargo.toml

@@ -22,3 +22,5 @@ visioncortex.workspace = true
 # Rasterize-and-diff equivalence tests (stacked vs mosaic). Test-only; not
 # compiled for wasm targets, so the library stays wasm-safe.
 resvg = "0.45"
+# Decode the sample photo for the spline-fitting regression test. Test-only.
+image = { version = "0.25", default-features = false, features = ["jpeg"] }