Render same-document SVG URL clips - #510

Problem

PR #509 made clip-path: path(...) render, but unchanged web content can also reference an SVG clipPath in the current document. WebScene preserved url(#fragment) as a computed value without applying it to retained pixels.

Change

• resolve fragment-only references to a connected clipPath in the same document and shadow scope
• flatten bounded direct path, rect, circle, ellipse, and polygon geometry into the existing retained path resource
• add objectBoundingBox scaling to the shared kind-12 ABI and Skia/Flutter presenters
• compare the reduced string index and all visual metadata during scene equality
• fail closed for missing, detached, malformed, external, transformed, nested, or oversized resources
• add native, Skia, and browser-contract regressions plus explicit limits and release gates

This is the consumer stacked on merged provider PR #509.

Closes #504

Validation

git diff --check passed. Authored tests were not executed under the current implementation-throughput directive; skipped runtime, pixel, performance, memory, package, and CI gates are recorded in docs/validation/css-url-clip-20260919.md.