refactor(benchmarks): isolate workload and conformance state

[thejustinwalsh]

Summary

• move Icon Grid virtual-window, recycle, pan/autopan, smoothing, refresh, visibility, and metrics state into a per-mount workload instance
• explicitly initialize the shared scene transform for every workload mount so outgoing marquee/pan state cannot pollute the next workload
• extract Main/Presentation composition, runtime control binding, benchmark surface chrome, and bake progress into named React modules
• route MTSDF and Slug sampling/source-outline targets through a common borrowed-renderer conformance session while retaining renderer-private resources behind adapters
• split complete MTSDF artifact authentication into per-artifact tests without weakening byte, hash, GLB, glyph, page, or texture-allocation assertions

Evidence

• mise exec -- pnpm benchmarks:check
  • 53 test files / 301 tests
  • TypeScript, OXLint, Oxfmt, script tests, size gates, and production build passed
• mise exec -- pnpm benchmarks:check:react-doctor
  • React Doctor 0.7.2 reported no issues
• mise exec -- pnpm --filter @pmndrs/text-benchmarks test:presentation-workloads
  • all 42 Bitmap/MTSDF/Slug × WebGPU/WebGL2 × seven-workload cells rendered visible pixels
  • exactly one renderer remained alive per lane
  • the first run exposed and distinguished Text Ladder scene-transform pollution; the corrected rerun passed every lane
• mise exec -- pnpm --filter @pmndrs/text-benchmarks test:headless
  • all 19 isolated conformance scenarios passed
  • MTSDF/Slug sampling and source-outline targets passed through the new session adapters
• mise exec -- pnpm docs:check
  • OKF v0.2: 0 errors, 0 warnings

Boundaries

• The route host still owns renderer, canvas, RAF, GPU timer, font transactions, generic Text readiness/attachment/disposal, and telemetry history.
• Raster adapters forward the exact borrowed renderer and abort signal; they do not expose private raster resources or create a parallel renderer.
• Workload modules remain app-private public-API examples; no package API was added.

Stacked on #21.