Implement CSS will-change stacking contexts

[wieslawsoltes]

Problem

Unchanged Code OSS has 16 will-change declarations across 12 CSS files. WebScene ignored them, so transform/opacity animation surfaces did not establish their required stacking context before the real property changed.

Change

• generate native and managed identity, validation, and CSSOM for auto or a bounded comma-separated identifier list
• retain the normalized authored list in cold textual state and derive one packed stacking-context bit
• reuse the existing retained atomic paint/hit predicate without preallocating a compositor layer
• keep mutation paint-only with no layout, visual-tree node, timer, or frame demand
• add product-neutral CSSOM/hit-order coverage and a 4,096-node zero-layout performance gate

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 #800