refactor(benchmarks): colocate slug role scenes

[thejustinwalsh]

Summary

• move Slug release-role scene definitions from the renderer directory beside the conformance capture that owns them
• update the capture script and URL-loaded browser probe to resolve the target-owned scene module
• add a boundary regression rejecting the obsolete renderer path
• keep reusable live renderer infrastructure free of conformance scenario policy

Verification

• mise exec -- pnpm benchmarks:check
• 60 files / 314 unit tests plus 4 script tests
• complete 36-observation Slug role matrix across WebGPU, WebGL2, DPR 1, and DPR 2 with unchanged visual hashes
• strict TypeScript, OXLint, Oxfmt, package-size measurement, and production build
• mise exec -- pnpm docs:check: 0 errors, 0 warnings

Stack

Depends on #29.