Emit bounded retained backdrop filters

[wieslawsoltes]

Problem

Unchanged Code OSS uses backdrop blur on onboarding and blur plus saturation in chat. WebScene stored the CSS value but emitted no retained operation, so presenters could not sample already-painted DOM, canvas, and GPU content.

Change

• canonicalize the WebKit alias and expose generated property metadata
• emit one bounded, versioned kind-48 command before the element background
• preserve authored blur/saturation order with capped sigma and resource validation
• force backdrop boxes into the foreground phase so they follow retained canvas layers
• propagate localized damage when earlier sampled pixels change
• project exact Skia backdrop layers in Avalonia 12 and bounded replay in Flutter
• document the ABI and author native/CSS regressions for 4,096 effects and the Code OSS sequence

This is the WebScene provider/reference-presenter part of #503. SceneTech/AppScene#174 owns the packaged native Skia/Graphite consumer.

Validation

Per the current integration fast-merge directive, no build, test, WPT, runtime, pixel, performance, memory, package, or complete CI gate was run. git diff --check passed. The authored gates remain unexecuted and are documented in the issue and validation note.