perf(icons): cache tint color resolution

[JustYannicc]

What changed

• Added bounded caches for CSS tint normalization, validation, parsed RGB values, root CSS variable reads, and readable tint results.
• Added theme/style invalidation so dark/light switches and root style changes do not leave stale readable colors.
• Added a focused tint cache regression/measurement harness.

Why

Icon tint and readable-color helpers run during list/grid/accessory rendering. Repeated DOM element creation and computed-style reads are avoidable for repeated static tint colors.

Compatibility impact

• Preserves named CSS colors, hex normalization, invalid-color handling, object light/dark tint selection, theme changes, and minimum-contrast adjustment.
• Dynamic CSS expressions such as var(...) are not cached as parsed/readable values, so they keep recomputing like the previous behavior.

Metrics

10k repeated calls on the focused harness:

• resolveTintColor string: 10,000 element creations before -> 1 after.
• object light/dark tint: 20,000 element creations before -> 2 after.
• resolveReadableTintColor: 20,000 element creations and 20,000 getComputedStyle calls before -> 2 and 2 after.

How tested

• node scripts/test-icon-tint-cache.mjs
• CLANG_MODULE_CACHE_PATH=/private/tmp/supercmd-clang-module-cache node --test 'scripts/test-*.mjs'
• LSP diagnostics on edited files: no errors

Stack validation

Clean PR branch created from origin/main and cherry-picked only this fix. No integration-stack dependency.

[JustYannicc]

Superseded by consolidated upstream PR #555.