refactor(benchmarks): define canonical live text scenes

[thejustinwalsh]

Summary

• define a workload-owned live-text scene contract and registry
• express Benchmark Ipsum and Advanced Shaping as canonical authored scene definitions
• derive Advanced Shaping font fixtures from each authored script case so text, language, features, direction, and font cannot drift apart
• leave the route responsible only for runtime sizing and renderer adaptation

Verification

• mise exec -- pnpm benchmark:check — 55 files, 307 tests; lint, format, types, scripts, size checks, and production build passed
• focused live-scene tests — 19 tests passed
• mise exec -- pnpm docs:check — 0 errors, 0 warnings
• workload boundary tests confirm authored workload modules do not import renderer internals

Stacked on #23.