fix(search): observe application changes outside search lock

[jewei]

Summary

Addresses second-review finding A: application watcher metadata observation now occurs before taking the search mutex. Only NotFound establishes absence; other metadata failures do not fabricate removals. In-memory comparison remains short, and bounded path collection and burst coalescing are preserved.

Updates the architecture overview for the latest-waiting queue, independent cancellation channel, cooperative request budget, publication lock order, and window-scoped command permissions. Avoids logging observed application paths.

Verification

• Full local verification: 44 checker tests, 234 Rust passed (7 default ignores), 219 mocked-IPC browser tests, formatting/types/Clippy/build and both profiling-harness modes passed.
• Latch-controlled regression drives the same orchestration as the production watcher and completes an actual SearchManager query while metadata is blocked.
• Negative mutation restoring the old lock scope failed at the blocked-search assertion; restored code passed. Workers release/join before assertions, including failure paths.
• Install/removal, rename, metadata-error and bounded event-burst coverage added.
• Two fresh independent subagent reviews approved da0345a: 8a9cf0e0-0ed1-422 and 11e4aaa2-3498-4da. Subagent reviews are not human GitHub approvals.
• Current-head Checks run 36394204863 passed all three platform jobs, installed Windows/Linux native journeys, and protected Required checks.
• Native source c45022e59dc1bc96c6d56c204e62312d967afc48 has reviewed tree e51f26e6268e7312ef3e972c44b94e3a1cf16f64; per-platform build/test identities match, source remained clean, cleanup completed, and all 100 schema-2 timing samples were retained (25/scenario).
• Windows package SHA-256: 82acd528bdf48d4370331c144e0826d51244917a8a814f461c2d721e2a78bab4.
• Linux package SHA-256: cdacbd8b64fc09c90ea201bcce2f74ce5ccd16adf60f53683a8ed2fe4d405995.

Native latency impact is not inferred from the latch test. No macOS/Wayland native or physical-paint proof is claimed, and no production release is published.