perf(chat): move history projection to worker

[beruro]

Summary

• move large ChatHistory pipeline and group projection work into a module Web Worker
• share one pure projection core between Worker execution and main-thread fallback
• add versioned snapshot/delta protocol with stale-response, resync, timeout, and multi-consumer protection
• keep small histories on the main thread and preserve/loading-state existing projections while Worker updates are pending
• replace render-time full-array virtual-list keys with deterministic layout digests
• document EventStore, Worker projection, and React ownership boundaries

Verification

• pnpm exec vitest run src/engines/ChatPanel/ChatHistory/projection/__tests__ src/engines/ChatPanel/ChatHistory/chatItemPipeline/__tests__ src/engines/ChatPanel/ChatHistory/hooks/__tests__/useChatGroups.test.ts src/engines/ChatPanel/ChatHistory/hooks/__tests__/useChatTurnPagination.test.ts — 11 files, 235 tests passed
• scoped ESLint with --max-warnings=0 — passed
• pnpm exec tsc --noEmit --pretty false — passed
• pnpm exec webpack --mode production --no-cache — passed
• git diff origin/develop...HEAD --check — passed

Performance validation note

The implementation includes deterministic 10,000-event coverage and a 50,000-event pressure fixture. Real Tauri runtime measurements for INP, main-thread P95, structured-clone cost, and steady-state memory remain follow-up acceptance work with production session data.