perf(benchmarks): close retained Presentation workloads

[thejustinwalsh]

Summary

• publish resident width, font-size, font-fixture, and Dynamic Layout changes through the Three.js object lifecycle without benchmark-side warm readiness waits
• pre-shape the complete 16-word Zoom Text corpus during cold scene construction and keep Icon Grid recycling synchronous with retained scratch storage
• remove recurring workload telemetry and animation allocations, while preserving the full 1,402-icon traversal
• run explicit conformance and finite captures exclusively through the persistent route renderer
• add dual-backend Presentation workload/demo matrices, reproducible screenshots, bounded Icon Grid CPU/trace profiling, and a scored pinned React Doctor gate

Product evidence

• Bitmap, MTSDF, and Slug each completed all seven workloads sequentially on WebGPU and forced WebGL with visible ink, one canvas, one renderer, retained backend identity, DPR/technique handoffs, and no browser warnings or errors
• both 60-second demos completed their authored sequence, included both Icon Grid passes, and ended on Off-axis / 3D; Icon Grid measured 60.02 FPS on both backends
• the 27-cell WebGPU cadence sweep held 60.0–60.2 FPS with zero slow frames in every MTSDF/Slug cell and every Bitmap cell except Paragraph Stress
• the 20-second Bitmap Icon Grid trace recycled 660 assignments across 31 windows at 18.60 ms p95, 18.64 ms maximum, and zero frames over 20 ms; measured GC events stayed below 2.6 ms
• exact npx react-doctor@0.7.2 and the pinned package gate report no issues and 100/100 with warning-level blocking enabled
• the retained screenshot workflow captures actual text for all seven workloads on both WebGPU and forced WebGL (14 images)
• the complete repository check, benchmark check, dual-backend comparison/capture recovery probe, package-size gate, packed consumer, and OKF validator pass locally

Deliberate limit

Bitmap Paragraph Stress renders 11,510 glyphs through 1,120 draws and measured 58.8 FPS with three frames over 20 ms. This PR does not weaken that workload or add renderer-wide batching across separate Text objects; batching remains outside item 10.5 and belongs in measured release follow-up work.

Stack

Depends on #15. Closes roadmap items 10.5 and 10.6 after the full signed stack passes CI.