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.

Cargo.toml

@@ -1,7 +1,28 @@
 [workspace]
 
 members = [
+    "crates/vtracer",
+    "crates/vtracer-cli",
+]
+
+# The pre-1.0 crates are kept in the tree for git history but are no longer
+# part of the build. They are replaced by the crates/ workspace above.
+exclude = [
     "cmdapp",
     "webapp",
 ]
-resolver = "2"
+
+resolver = "2"
+
+[workspace.package]
+version = "1.0.0-alpha.1"
+authors = ["Chris Tsang <chris.2y3@outlook.com>"]
+edition = "2021"
+license = "MIT OR Apache-2.0"
+homepage = "http://www.visioncortex.org/vtracer"
+repository = "https://github.com/visioncortex/vtracer/"
+
+[workspace.dependencies]
+# visioncortex 0.9.0 is currently unreleased; developed against the local
+# checkout. Releases will pin a published 0.9.x.
+visioncortex = { version = "0.9", path = "../visioncortex" }