refactor(benchmarks): isolate low-level targets

[thejustinwalsh]

Summary

• move external raster and React reconciliation proofs into benchmark/targets/product
• move realtime MTSDF/Slug comparison and runtime-fallback capture into benchmark/targets/conformance/raster
• place pure CPU raster and source-outline oracles under benchmark/low-level/raster
• update conformance surfaces and registries to retain literal lazy target imports
• enforce the dependency direction by rejecting renderer imports of executable benchmark targets

Verification

• mise exec -- pnpm benchmarks:check — 56 files, 311 tests; lint, formatting, strict types, size budgets, and production build passed
• realtime MTSDF/Slug comparison passed success/capture/navigation recovery on WebGPU and forced WebGL2
• external raster product proof passed deterministic retained frames on WebGPU and forced WebGL2
• all 19 isolated headless conformance scenarios passed
• mise exec -- pnpm docs:check — 0 errors, 0 warnings

Stacked on #25.