Scan engine QoL: exclusions, streaming, cancel-keeps-partial, pause/resume, rate display

[RasputinKaiser]

Summary

• Folder exclusion list: ScanOptions.excludeEnabled/excludedPathComponents/excludedAbsolutePrefixes, wired into Settings (toggle + add/remove list, seeded with node_modules, .git, ~/Library/Caches, ~/Library/Application Support) and an "Exclude This Folder" context-menu action in StorageItemTable, TreeExplorerView, CleanupReviewView, and DuplicateCandidatesView (which targets the containing folder, since duplicate rows are always files).
• Cancel now keeps partial results instead of discarding them; StorageScan.isPartial marks in-progress snapshots.
• Live streaming: FileSystemScanner.scan(...) gains an onSnapshot callback fired at the same throttled cadence as progress, publishing ranked lists into session.scan while a scan is running.
• Rate/elapsed display: ScanProgress.itemsPerSecond (EMA-smoothed in ScanStore), plus an elapsed-time readout in the sidebar footer.
• In-memory pause/resume: ScanCancellation rewritten on NSCondition with pause()/resume()/waitIfPaused(); a toolbar Pause/Resume button next to Cancel.

Notable fixes made during self-review

• Fixed a one-tick lag where the scanner's onSnapshot fired after progress inside emitProgressLocked, so ScanStore's snapshot/progress pairing was always one throttle-tick stale — reordered so onSnapshot fires first.
• Standardized the paths stored by excludeFolder/excludeFolder(atPath:) to match FileSystemScanner.isExcluded's comparison (both now use standardizedFileURL.path), so exclusions set from a symlinked-ancestor path still match on rescan.
• Gated the "Exclude This Folder" context-menu action on !store.isScanning in all four views — it routes through rescan(), which no-ops silently while a scan is already in flight (reachable now that streaming makes store.scan non-nil mid-scan).
• Flagged (documented, not solved) a known thread-pool-exhaustion risk: waitIfPaused() blocks inside DispatchQueue.concurrentPerform closures, so pausing a very wide directory scan can approach GCD's shared worker-pool ceiling.

Test plan

• swift test — 184 tests, 3 consecutive full-suite runs green
• swift build and swift build -c release — both clean
• New tests: exclusion (name/prefix/nested/toggle-off), snapshot() mid-walk, pause/resume blocking + cancel-while-paused, ScanSession.canPauseScan/canResumeScan guards, ScanStore cancel-keeps-partial-snapshot
• 5-angle automated code review (correctness × 3, cleanup, conventions) run against the diff; confirmed findings fixed, documented/deferred findings noted above

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