Render centered CSS circle clip paths in retained scenes #449

Parent: #256

Problem

Retained clip-path painting currently supports rectangular inset(...) only. The computed-value layer accepts circle shapes, but scene emission drops them, so the element and its descendants paint outside the authored circle.

Focused implementation

• Parse centered circle(<length-or-percentage>) from retained effect values.
• Resolve percentages against the CSS normalized diagonal.
• Emit the existing balanced rounded clip commands around the element and descendant paint.
• Reuse scene kinds 12/13 and their four existing radius fields; do not change the scene ABI or presenters.
• Leave positioned circles, radius keywords, ellipses, polygons, paths, masks, filters, and backdrop effects to later Render CSS masks, filters, clip paths, and backdrop effects in retained scenes #256 slices.

The work starts from exact WebScene main 4ea2feebd0dd7f2f08dfa1a8bfbbe1ca5f3d6c3b and will be rebased onto current main before review.

Validation debt

Implementation throughput is the current priority. Tests, browser/native pixel contracts, performance and lifecycle benchmarks, hosted checks, and CI monitoring are explicitly deferred and must be completed before a broader support or release claim.