[0.44 internal alpha.4] Make the critical rendering fast path the default

[SisyphusZheng]

Part of #1155. Target: internal alpha.4 workspace.\n\n## Context

A Lighthouse audit of the www site (mobile, Moto G + 4G throttling, production build) scored performance 84. Every deduction traced to asset discipline, not the rendering architecture — TBT ≈ 0 confirms the runtime side is clean. The findings expose a framework gap: openElement currently has no opinion on the critical rendering path. Every site must independently rediscover font preload, blocking-CSS discipline, and third-party SPOFs.

Measured evidence (prod build, getting-started):

• CLS 0.195 (limit 0.1): four @font-face with font-display: swap and no preload → text reflows on font arrival (.article-content 0.109 + article.main 0.085).
• LCP 2.7s (limit 2.5s): ~950ms render delay includes a render-blocking Prism theme CSS fetched from a third-party CDN origin.
• Injected token style block ships unminified (~6 KiB avoidable).
• dev-server Lighthouse scores are ~20 points worse than production with no tooling guidance, causing false alarms.

Proposal (candidates, each can land independently)

1. Critical-assets convention: a declarative way to mark fonts/styles as preload/critical in openElement() config; the build emits correct <link rel='preload' as='font' crossorigin> / inline critical CSS instead of every site hand-writing headFragments.
2. Performance-aware inject policy: the head inject already has a security allow-list (scripts rejected, base stripped); extend it to flag or reject cross-origin render-blocking stylesheets and sync external scripts, with an explicit escape hatch.
3. Build-time minification of injected inline styles (the colorTokensStyle-style blocks).
4. Perf gate task: a repo-task level Lighthouse/budget check against the production build (CLS ≤ 0.1, TBT budget, 'errors-in-console' = 0), plus docs guidance that performance scores are only meaningful against dist.

Non-goals

Streaming SSR does not belong here: it does not change render-blocking semantics and is a no-op for static SSG output. This issue is about static asset discipline for the prerendered path.

Site-level fixes for www land separately (font preload, self-hosted/inlined Prism, theme-init inlining). Component a11y findings tracked in the sibling issue.

Workspace boundary

Write only the alpha.4 paths assigned in tools/config/v044-alpha-workspaces.json. This issue starts with the workspace and does not wait for another Alpha workspace.

[SisyphusZheng]

Site-level finding while fixing www: the framework rejects <script> in inject.headFragments (H-04, deliberate) and inject.scripts only accepts URL entries — there is no inline-script channel. www's theme-init.js (1.5KB anti-FOUC bootstrap, must run pre-paint) therefore has to stay an external sync request. A validated inline: string script option (or a blessed pre-paint script hook) would close this; recording it here so it lands inside the critical-assets convention rather than as a site workaround.

[SisyphusZheng]

Reactivated under ADR-0143 as a cross-cutting, RC-relevant convention issue. Implementation is coordinated with #1171 (reachability-based zero-runtime delivery) and #1176 (measured performance budgets); this issue owns the author-facing fast-path conventions, not a second renderer optimization path.