perf: stream public profile content

[sarthakagrawal927]

Summary

• stream a stable, accessible public-profile shell while D1 data resolves
• remove the duplicate full-profile query from the slug layout
• keep resolved stream content visible while preserving motion polish
• update performance and product truth plus responsive design evidence

Validation

• pnpm typecheck
• pnpm lint
• pnpm test (17 files, 78 tests)
• pnpm build
• pnpm docs:check (0 errors; 8 pre-existing archive warnings)
• design-workflow check
• git diff --check
• throttled Slow 3G browser pass at 390px; responsive evidence at 390/768/1440

Refs #28

This completes the documented source-level public-profile TTFB quick wins. Fresh production p50/p75 measurement remains a separate post-deploy evidence step; this PR does not deploy.