perf(runtime): optimize scratch-render readbacks

[kubohiroya]

Closes #564

Depends on:

• Optimize silhouette readback canvas TurboWarp/scratch-render#21
• perf(web): optimize Packager canvas readbacks tm-kamishibai-samples#115

Summary

• pin scratch-render to reviewed fork commit 1fa6cc7d23e12aabf8db16e8e3ce400538f44165
• mark only the Silhouette and TextBubble readback contexts as willReadFrequently
• patch the same two exact templates in Packager 3.13.0 Plain HTML and zip-one-asset outputs
• fail closed on Packager template drift; no global Canvas monkey patch
• expose the audited Packager/fork/upstream contract and regression tests

Verification

• upstream fork: npm run lint, npm run build
• targeted Packager tests: 12/12
• pnpm verify:full
  • Node 1,240/1,240
  • quick E2E 50/50
  • real Chromium 13/13
  • SB3 publication snapshot, lint, format, typecheck, site, npm package smoke
• samples: Node 34/34, site/tutorial verify, Chromium/WebKit/DSL 4.0 browser suites
• performance: 2,000 picks median 1.0ms before and after (ratio 1.00)

Rollback

Revert this PR to restore TurboWarp/scratch-render@a67f7c9, revert samples PR #115, and regenerate the Web artifacts. Do not fall back to a Canvas prototype monkey patch.