Contain retained scroll chaining with overscroll-behavior

[wieslawsoltes]

Closes #793.

Problem

Unchanged Code OSS uses 12 overscroll-behavior declarations to suppress workbench bounce and contain Markdown, Chat, carousel, voice-input, sessions/mobile, and overlay scrolling. WebScene did not expose the property and chained every exhausted nested wheel target into its ancestors.

Change

• add generated native/managed identities and CSSOM accessors for the shorthand and both physical-axis longhands;
• parse and serialize auto, contain, and none, including one/two-value shorthand, global keywords, mutation, removal, and cascade fallback;
• retain two compact policies in the existing cold textual style allocation;
• stop each exhausted wheel axis at a qualifying scroll container for contain and none, while preserving auto, the other axis, root viewport ownership, cancellation, discrete/precise scheduling, and scroll events;
• keep policy mutation interaction-only with no layout or scene publication;
• add direct CSSOM/WPT, nested wheel, lifecycle, and 4,096-node performance gates plus the qualification record.

WebScene has no rubber-band affordance, so contain and none intentionally share scroll-chain stopping in this bounded slice.

Evidence

git diff --check passed. Per the active fast implementation direction, no build, test, WPT, visual, package, memory, lifecycle, accessibility, or benchmark command was executed; those authored gates remain zero evidence pending consolidated validation.