Expand single-layer CSS mask shorthands

[wieslawsoltes]

Problem and result

Unchanged Code OSS authors many icon and fade masks through mask and -webkit-mask, while WebScene previously retained only separately-authored longhands. Those declarations never reached the retained mask path.

This provider change:

• exposes standard and WebKit-prefixed mask property identities;
• canonicalizes prefixed declarations to standard effective properties;
• expands bounded single-layer URL/gradient shorthand forms into image, position, size, repeat, composite, and mode longhands before cascade application;
• resets omitted components to initial values and supports whole-shorthand variable resolution;
• adds browser-shaped mask-mode initial/value handling;
• rejects multiple layers, geometry boxes, malformed geometry, and unsupported forms as one unit;
• authors native, CSSOM, alias, mutation, and retained-command regression coverage.

URL-backed SVG projection is implemented by dependent PR #502 for #500. GitHub displays #502 as a stack child because its base is this provider branch. Merge this PR first, retarget #502 to main, then merge #502.

Validation

git diff --check passes and generated metadata was refreshed. The first CI attempt found a stale expected property count; tip abf311e5 updates 105 to 106. Other observed failures were runner DNS checkout failures and pre-existing mainline native footprint / aspect_ratio_* compile errors. Replacement workflows were canceled, so no post-fix pass is claimed. Per the current throughput directive, builds, native/browser tests, WPT, pixels, performance, memory, package, and CI validation were intentionally not run; the cumulative debt is recorded in #499, #256, and #235.

Closes #499