fix: watcher leak, registry write races, and stalled scans

[anbturki]

Fixes the three code-analysis findings filed after the v0.9.1 sync work. Each was proven with a test that fails against the current code, and the registry fix was additionally measured end to end with real concurrent sidecar processes.

Changes

Watcher leak (useTabs) - a tab's watch() could resolve after its path changed. The old id-based guard still passed, so the real unwatch was assigned to an orphaned slot and the FSEvents stream leaked for the session. Now the resolved attach checks whether the map still holds the exact slot it created, which covers path swaps, tab close, unmount, and rapid successive swaps in one comparison. The rejection path no longer evicts the live watcher that replaced it.

Registry atomicity (core) - concurrent MCP sidecars each read the registry, added an entry, and wrote the whole file back, silently dropping each other's workspaces. Now an exclusive advisory lock spans the load-modify-save, and writes go through a temp file plus rename. The lock is on a sidecar path deliberately: the rename swaps the registry inode, so a lock on the registry itself would leave writers holding different inodes and never contending.

Stalled scans (scan) - a scan on unreachable storage never settles, pinning the workspace spinner. It now fails after 60s without progress, an inactivity window rather than a fixed deadline so healthy large scans are never cut off.

Evidence

Registry, real concurrent sidecar processes (not threads):

build	24 procs	48 procs
pre-fix	12, 14, 22 of 24	41, 43, 43, 45 of 48
fixed	24, 24, 24	48, 48, 48

Zero leftover temp files across all fixed runs.

Scanner load test on synthetic workspaces:

• 25,000 files: ~400ms, max gap between progress events 100-117ms.
• 55,000 files (over the 50k cap): capped at 50,000 with truncated=true, ~1.9-3.3s, max gap 122ms. The 60s inactivity window therefore has roughly a 490x margin over the worst observed healthy gap.
• Adversarial input (oversize file, symlink loop, permission-denied directory): all three counted as skipped, all real files still found, and the symlink loop did not hang the walk.

Flakiness: registry concurrency test 12/12, full frontend suite 5/5.

Suite: 135 frontend tests, 81 core tests, tsc --noEmit clean, cargo clippy --workspace --all-targets clean.

Fixes #25
Fixes #26
Refs #27