Render CSS path() clip geometry

[wieslawsoltes]

Problem

Unchanged Code OSS uses clip-path: path(...), but WebScene retained scenes only projected inset, circle, ellipse, and polygon geometry. Valid path clips therefore reached computed style without affecting pixels.

Change

• parse one bounded quoted SVG path with optional evenodd/nonzero
• retain border-box-relative coordinates and fill rule in the existing kind-12 ABI
• project the same command through Skia and Flutter
• compare path text plus visual metadata during scene equality
• fail closed on malformed/unparseable paths
• add native, Skia, and browser-contract regressions and explicit release gates

This is the provider slice of #504. Same-document url(#clip) resolution remains the consumer slice, so #504 stays open.

Validation

git diff --check passed. The authored native/backend/browser tests were not executed under the current implementation-throughput directive; the skipped gates are recorded in docs/validation/css-path-clip-20260919.md.