[wieslawsoltes]

Problem and result

Code OSS ring masks use standard exclude and WebKit xor, while the retained mask path accepted only additive layers. This change preserves authored CSSOM spelling, normalizes both values to an explicit retained exclude operation, and adds webscene-mask-v3 with a per-layer composite field.

Avalonia and Flutter now paint the bottom mask first, combine upper layers with source-over or xor, and apply the completed alpha surface to the element once. V2 additive resources remain compatible; subtract/intersect and malformed values fail closed. Native, portable CSS/WPT, alias, ordering, and metadata contracts are authored.

AppScene packaged consumption is tracked by SceneTech/AppScene#178.

Closes #506

Validation

• git diff --check passed.
• Builds, tests, WPT execution, pixels, performance, memory, lifecycle, packaging, and full CI were intentionally deferred under the current fast-merge directive.