refactor(benchmarks): organize workload and target hierarchy

[thejustinwalsh]

Summary

• make every retained workload definition own its create, layout, animation, and retained-configuration phases
• move the Benchmark Ipsum corpus and Advanced Shaping timeline beside the authored workload scenes
• move the self-contained Advanced Shaping conformance implementation under the explicit low-level target hierarchy
• preserve renderer ownership, RAF, telemetry, transactional publication, and literal selected-target lazy loading in the persistent host

Evidence

• mise exec -- pnpm benchmarks:check
  • 51 test files / 285 tests
  • lint, Oxfmt, TypeScript, script tests, size gates, and production build passed
• mise exec -- pnpm docs:check
  • OKF v0.2: 0 errors, 0 warnings
• Chromium Presentation matrix
  • all 42 Bitmap/MTSDF/Slug × WebGPU/WebGL2 × workload cells rendered visible pixels
  • exactly one renderer remained alive per lane
• isolated headless conformance
  • all 19 scenarios passed
  • Advanced Shaping completed three exact 68-frame timelines with zero warm readiness waits

Boundaries

• Icon Grid's active virtual pool remains host-owned and is the next stateful workload-isolation layer.
• Coupled MTSDF and Slug targets remain in their current renderer modules until a shared conformance-session adapter can preserve warm load/run/dispose behavior and borrowed-renderer ownership.

Stacked on #20.