Implement pointer-driven CSS :active state with bounded invalidation #478

Problem

WebScene parses :active, but the compound matcher has no active-state branch. Unchanged VS Code OSS uses :active extensively for buttons, tabs, list rows, actions, and other pressed feedback, so pointer presses never expose the expected style and visual comparison diverges.

Proposed implementation

• Add a host-independent pressed target to selector interaction state.
• Match :active on the pressed target and its inclusive ancestor chain, isolated to the current document tree.
• Index :active selector dependencies separately from :hover so pointer transitions recascade only affected subjects, descendants, siblings, and custom-property consumers.
• Enter active state before pointerdown/mousedown dispatch and clear it before pointerup/mouseup, cancellation, teardown, or navigation so synchronous computed-style reads see browser-shaped state.
• Keep synthetic events and HTMLElement.click() from changing user-interaction state.

Acceptance and deferred validation

The implementation must retain bounded invalidation and support direct, ancestor, descendant, sibling, functional-pseudo, pseudo-element, and custom-property selectors. Browser/WPT/native, performance, memory, lifecycle, exact SDK/package, and CI validation are deliberately deferred under the current implementation-throughput directive and remain required before release promotion.

Parent: #237. Coordination: #235, #242, #243, and vscode-demo#1.