Rewrite into a vectorization framework (pillars 1–4)

Replace the 0.6.x single-pipeline crate with a stage-based framework, per
docs/design/. Implements Motivation pillars 1–4 (frontend, curve fitting,
color fitting, optimizer); mosaic (5) and bindings are deferred.

Workspace:
- crates/vtracer      — the framework library (wasm-safe, no I/O)
- crates/vtracer-cli  — thin CLI wrapper (clap 4 + image I/O)
- cmdapp/ and webapp/ excluded from the workspace (git-preserved)

Stages behind object-safe traits, composed by a Pipeline driver:
- Frontend: ColorClusterFrontend (+ transparency keying), BinaryFrontend
- CurveFitter: Pixel / Polygon / Spline (region tracing via visioncortex)
- ColorFitter: Identity, FixedPalette (OKLab-nearest), AutoQuantize
  (area-weighted median cut), MergeAdjacent
- OptimizerPass: QuantizePass, SimplifyPass
- SvgWriter: relative/absolute shortest encoding, H/V/S shorthands,
  compact number formatting, <g fill> grouping

visioncortex is a path dependency on the local 0.9.0 checkout.

Verified: 14 unit/integration tests pass; framework builds for
wasm32-unknown-unknown; CLI output renders faithfully via rsvg.

.gitignore

@@ -1,4 +1,5 @@
 target
 Cargo.lock
 *.sublime*
-.vscode
+.vscode
+.DS_Store