refactor: compress language identity color palette

[limityan]

Summary

• Compress languageIdentityAccents from per-language/file colors into an 8-slot category palette.
• Compress the shared Prism syntax palette into role-based light/dark palettes while keeping all exported syntax scheme keys unchanged.
• Keep all exported built-in language keys and code-snippet accent keys unchanged while routing snippet accents through the same registry.
• Lower theme governance baselines and update theme-token-optimization.md with the new audit snapshot and consumer contracts.

Impact and Risk

• User-visible language/file identity colors are intentionally coarser. Exact identification still comes from language/file key, label, icon type, extension, and filename; color is not used as the sole identifier.
• Syntax highlighting is intentionally coarser for low-risk roles: light string/number now share a literal blue. After product-design review, light punctuation remains distinct from comments and dark tag/property remain distinct to preserve 12px code preview and JSX/HTML adjacent-token readability.
• No Rust/theme-service ownership boundary changes. This stays in web-ui TS-owned specialized color domains.
• Dynamic key risk was checked by comparing exported key sets against the base: BUILTIN_LANGUAGE_ACCENTS remains 48 keys, CODE_SNIPPET_LANGUAGE_ACCENTS remains 10 keys, and Prism light/dark scheme keys remain 9 each.

Review Follow-up

• Subagent architect review found no blocking issues.
• Subagent product-design review found Prism readability risks; fixed by restoring light punctuation contrast and dark tag/property separation, then updating baseline/docs.

Metrics

Metric	Before	After
web-ui color occurrences	599	553
web-ui unique colors	395	356
language identity occurrences	52	8
language identity unique colors	50	8
syntax occurrences	18	16
syntax unique colors	17	16
exception unique color baseline	229	190

Verification

• pnpm run theme:color-audit:all
• pnpm run theme:color-audit:test
• pnpm run type-check:web
• pnpm run lint:web
• pnpm --dir src/web-ui run test:run src/shared/context-system/core/types/CodeSnippetContextImpl.test.ts
• pnpm --dir src/web-ui run test:run
• git diff --check