fix: clear persisted performance snapshot on Restore All and serialize baseline capture

[laurentiu021]

Two Performance Mode reversibility defects.

Changes

• Stale persisted baseline after Restore All — RestoreAllAsync cleared the in-memory _snapshot but never deleted performance-snapshot.json. Since EnsureSnapshotAsync seeds the baseline with LoadSnapshot(), the next Apply reloaded the now-reverted pre-restore state as its baseline, and a later Restore All could re-apply stale values. Added PerformanceService.DeleteSnapshot() (guarded like SaveSnapshot) and call it after a successful Restore All.
• Racy baseline capture — the six independent Apply commands each call EnsureSnapshotAsync, which does an unguarded _snapshot ??= Load() ?? Take(). Two running together could both see null and race the capture/save. Wrapped the load-modify in a SemaphoreSlim(1,1) so the baseline is captured exactly once. (Disposed in Dispose.)

Notes

• The broader "serialize ALL Apply mutations through OperationLockService" suggestion is a larger UX/behavior change deferred; this PR fixes the concrete data races without changing which buttons are enabled.

Build: 0 warnings / 0 errors. Version 1.20.41. (Stacks on #930; will rebase before merge.)