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

[beruro]

Problem

Repo/project/sidebar/benchmark loads can duplicate in-flight work, file-index and project caches lack complete invalidation paths, and hidden browser/webview work can retain CPU or memory beyond its active lifecycle.

Solution

• Coordinate equivalent loads with retryable single-flight ownership.
• Bound file-index caching and invalidate it from authoritative git watcher events.
• Suspend hidden webview/layout work and cap retained browser surfaces.
• Add explicit purge/invalidation paths for project caches.
• Validation: 49 focused Vitest tests, TypeScript typecheck, all changed-file ESLint, Rust search tests/rustfmt, and diff check against refactor(async): add scoped resource lifecycle #562.

Potential risks

• Depends on refactor(async): add scoped resource lifecycle #562 and must be retargeted to develop only after refactor(async): add scoped resource lifecycle #562 merges.
• This is a systematic performance-lifecycle sweep; cache invalidation, retry-after-owner-failure, and repeated open/close behavior are the highest-risk paths.
• Identity/scope keys and bounded retention must remain intact when new consumers are added.