perf(runtime): bound caches, scans, and background work

[beruro]

Summary

• coordinate duplicate repo/project/sidebar/benchmark loads and make failed owners retryable
• bound file-index caching and invalidate it from git watcher events
• suspend hidden webview/layout work and cap retained browser surfaces
• add explicit purge/invalidation paths for project caches

Stack

Depends only on #562 for latestScopedTask and visibility-aware polling. It is independent of the RPC/session-intent stack rooted at #512.

Validation

• 11 targeted Vitest files: 49 tests passed
• pnpm typecheck
• ESLint on all changed TS/TSX files
• cargo test -p search file: 9 passed
• rustfmt check on all changed Rust files
• git diff --check junyu/async-resource-lifecycle...HEAD

Performance guard

Lifecycle matrix covered active, hidden, unmounted, retry-after-failure, invalidation-during-build, and bounded-retention states. No unbounded timer, waiter, cache, or retained-webview growth is introduced.