feat(campaign): add eval fixture planning UX

[drewstone]

Summary

• add Vercel-style eval fixture loading/planning APIs on top of runCampaign
• fix campaign cache reuse by storing/checking per-cell manifestHash and dispatchRef
• add docs, changelog, and offline eval-fixtures quickstart example

Verification

• pnpm tsx examples/eval-fixtures-quickstart/index.ts
• pnpm vitest run tests/campaign/run-campaign.test.ts tests/campaign/fixtures.test.ts
• pnpm typecheck
• pnpm lint (passes with existing warnings outside touched files)
• pnpm test
• NODE_OPTIONS=--max-old-space-size=8192 pnpm build
• pnpm verify:package
• git diff --check
• git merge-tree --write-tree origin/main HEAD