refactor(benchmarks): route scenes through workload controllers

[thejustinwalsh]

Summary

• make every retained workload definition own typed app-private scene construction and layout hooks
• move Icon Grid layout, virtual-window calculation, assignment validation, recycled positioning, pan integration, and frame-delta smoothing beside its canonical Text example
• derive Main and Presentation descriptions, amount labels, font behavior, preload grouping, and pan/zoom policy from the typed catalog
• keep renderer, canvas, RAF, GPU timer, telemetry, font transactions, active pool ownership, and teardown in the persistent host

Why

The workload files already contained the canonical public Text examples, but the retained renderer still switched on workload IDs to construct and lay out them. The UI also repeated workload policy independently of the typed catalog. This layer makes the file hierarchy an executable boundary without inventing a public framework API or moving host lifecycle ownership into scene code.

Verification

• mise exec -- pnpm benchmarks:check
  • 51 test files / 285 tests
  • TypeScript, Oxlint, Oxfmt, script contracts, size gates, and production build pass
• focused workload/controller suite: 5 files / 64 tests
• 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 in every backend/technique lane
  • no reported console or page failure
• mise exec -- pnpm docs:check: zero conformance errors, producer-profile errors, or warnings
• all three commits are SSH-signed by Justin Walsh <contact.me@thejustinwalsh.com>

Stack

Depends on #19.