Render retained polygon clip paths

[wieslawsoltes]

Unchanged Code OSS uses a six-point clip-path: polygon(...) for the Markdown diff change indicator, but WebScene currently paints that pseudo-element as its full rectangular background. This change resolves bounded px/% polygon points once in the native scene emitter and clips them in Avalonia/Uno and Flutter through their existing SVG path support.

The scene ABI struct and version remain unchanged: clip-begin kind 12 reserves its top flag bit for an indexed path resource, and kind 13 remains the balanced end. Existing inset and circle commands retain zero flags and their rounded-rectangle geometry. The parser accepts default-fill-rule polygons with 3–64 point pairs; other units, fill-rule arguments, and other effect families remain under #256.

Validation debt: per the implementation-throughput priority, tests, browser/native pixel contracts, performance/lifecycle benchmarks, hosted checks, CI monitoring, and exact-package visual acceptance were skipped. git diff --check passes on source head bdcab003 rebased onto main a2f873e8.

Closes #456
Parent: #256