perf(playground): preload the initial sans font

[wiyco]

Summary

• Preload the emitted GeistVariableLatin-*.woff2 asset in generated playground HTML.
• Resolve the Vite hashed font asset at build-output time, including the correct relative URL for nested pages.
• Keep the mono font out of the initial preload path and allow disabling the preload with SVGO_PRELOAD_INITIAL_FONT=0 for A/B checks.

Validation

• pnpm run check:fix
• pnpm run test:coverage
• pnpm --filter ./apps/playground typecheck
• pnpm vitest run apps/playground/src/build/preload-font-build-output.test.ts apps/playground/src/entries/entrypoints.test.tsx
• pnpm --filter ./apps/playground build

Notes

Local preview timing showed the sans font request moving from a CSS-initiated start around 46.5ms to a link-initiated start around 10.6ms. I kept font-display: block unchanged: the swap check did not show a meaningful paint improvement after preload, and delayed-font checks showed visible text metric changes that could surface as FOUT.

[github-actions]

Code Metrics Report

Download coverage report: lcov-report

	main (441a7a4)	#5 (7e711ed)	+/-
Coverage	100.0%	100.0%	0.0%
Code to Test Ratio	1:1.8	1:1.8	-0.1
Test Execution Time	1s	1s	0s

Details

  |                     | main (441a7a4) | #5 (7e711ed) | +/-  |
  |---------------------|----------------|--------------|------|
  | Coverage            |         100.0% |       100.0% | 0.0% |
  |   Files             |             63 |           64 |   +1 |
  |   Lines             |           1323 |         1358 |  +35 |
+ |   Covered           |           1323 |         1358 |  +35 |
- | Code to Test Ratio  |          1:1.8 |        1:1.8 | -0.1 |
  |   Code              |           4265 |         4356 |  +91 |
+ |   Test              |           7948 |         8080 | +132 |
  | Test Execution Time |             1s |           1s |   0s |

Code coverage of files in pull request scope (0.0% → 100.0%)

Files	Coverage	+/-	Status
apps/playground/src/build/preload-font-build-output.ts	100.0%	+100.0%	added

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Reported by octocov