Honor user-select for retained text interaction

[wieslawsoltes]

Closes #791.

Problem

Unchanged Code OSS uses 276 user-select declarations across 94 files, but WebScene did not expose the property and ordinary pointer input never created document Selection state. Monaco, terminal, notebook, Chat, list, dialog, preview, and webview surfaces therefore could not rely on browser-shaped selection policy.

Change

• add canonical user-select plus WebKit/MS aliases to generated native and managed CSS metadata;
• cascade, reset, mutate, and serialize auto, text, none, and all from cold style state;
• create retained Selection ranges on pointer down/move/up with cancelable selectstart;
• honor none, explicit descendant text, and atomic all subtree selection;
• paint ordinary selections with system Highlight/HighlightText, with matching author ::selection declarations overriding the defaults;
• keep mutation paint-only and reuse existing Selection/Range, copy, and outbound-drag paths;
• add direct CSSOM/WPT, pointer interaction, lifecycle, and 4,096-node performance gates plus the qualification record.

Evidence

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