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.

crates/vtracer/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "vtracer"
+description = "A vectorization framework that converts raster images into vector graphics: pluggable frontends, curve fitters, color fitting, and output optimization."
+version.workspace = true
+authors.workspace = true
+edition.workspace = true
+license.workspace = true
+homepage.workspace = true
+repository.workspace = true
+categories = ["graphics", "computer-vision"]
+keywords = ["svg", "vectorization", "computer-graphics"]
+
+[lib]
+name = "vtracer"
+path = "src/lib.rs"
+
+[dependencies]
+visioncortex.workspace = true