refactor(benchmarks): remove standalone previews

[thejustinwalsh]

Summary

• remove the zero-consumer Bitmap, MTSDF, and Slug standalone preview constructors
• delete their duplicate renderer, RAF, GPU timer, telemetry, resize, and disposal lifecycles
• rename retained update/snapshot contracts and React viewport state from preview terminology to persistent scene terminology
• define persistent scene options directly and add a boundary regression rejecting preview entrypoints
• reduce the live Bitmap/MTSDF/Slug renderer chunks without changing package runtime artifacts

Verification

• mise exec -- pnpm benchmarks:check: 60 files / 315 tests, strict TypeScript, OXLint, Oxfmt, size measurement, production build
• mise exec -- pnpm benchmarks:test:presentation: all 42 sequential workload cells visible with one renderer
• complete WebGPU/WebGL2 timed demos returned to Off-axis / 3D at 59.92/60.02 Icon Grid FPS
• mise exec -- pnpm benchmarks:check:react-doctor: no issues
• mise exec -- pnpm docs:check: 0 errors, 0 warnings
• live renderer chunks: Bitmap 16.82/5.19 → 10.37/3.84 kB minified/gzip; MTSDF 9.02/3.32 → 8.37/3.12; Slug 9.26/3.37 → 8.19/3.09

Stack

Depends on #30.