Implement CSS isolation stacking contexts

[wieslawsoltes]

Problem

Unchanged Code OSS uses isolation:isolate on five retained surfaces. WebScene ignored the property, so a positive-z descendant could escape its intended paint group and appear or hit above later sibling UI.

Change

• generate native and managed CSS identity, validation, and CSSOM exposure for isolation:auto|isolate
• store the computed result in one packed style bit and feed the existing retained atomic stacking-context predicate
• keep CSSOM mutation paint-only because scene and hit-order traversal evaluates context ownership dynamically
• add product-neutral CSSOM/hit-order coverage and a 4,096-node zero-layout performance gate
• record the unchanged Code OSS trigger, bounded scope, and evidence debt

Validation

Per the active fast-merge direction, no build or test suite was run locally. Static source review, JSON parsing, generated-metadata review, and git diff --check pass. Authored gates remain unexecuted qualification debt.

Closes #795