v2.0 cleanup: remaining deprecated APIs & theme frontmatter relocation

[YunYouJun]

Tracking issue for cleanup deferred past 1.0 (these are not blockers for the 1.0 line; scheduled for the next major since removals require a major bump).

Context: the 1.0 deprecation sweep removed unocssPresets.uno, top-level ignoreDeadLinks, the color frontmatter type, the built-in APlayer/Meting widget (→ valaxy-addon-meting), and the legacy vite-ssg SSG engine (#706).

Remaining for v2.0

• Remove useCodePen (packages/valaxy/client/composables/widgets/codepen.ts). It only injects the CodePen embed script and is redundant with the documented useScriptTag('https://static.codepen.io/assets/embed/ei.js') one-liner (see docs/pages/guide/post.md → Scripts). Remove the widget + its frontmatter.codepen handling in ValaxyMd.vue; point users to the Scripts doc.
• Relocate theme presentation frontmatter to theme-yun. pageTitleClass / postTitleClass are theme-yun-only presentation fields still declared in core PageFrontMatter (packages/valaxy/types/frontmatter/page.ts). Establish a declare module 'valaxy' frontmatter augmentation pattern in theme-yun and move them there (deprecated color already removed from core). This needs new augmentation infra + verification that the typing surfaces to end-user .md/config — do it as a deliberate change, not a half-migration.

Optional / nice-to-have

• Critical CSS as an opt-in post-process. beasties was removed with vite-ssg; the default engine uses the FOUC guard instead. If Lighthouse/FCP demands it, add Critical CSS inlining as an opt-in (off by default) post-build step on the built-in engine (beasties operates on built HTML, no JSDOM needed). Never default-on (risk of missing rules → visual glitches).