M10: Tests for safety-critical error branches (8 subsystems) (closes #30)

[curtyo18]

Summary

Pure test additions covering safety-critical error branches across 8 subsystems. No production code changes.

Each branch in scope is the primary defense against silent data corruption in its area. They were uncovered at review baseline; this PR catches future regressions in the error paths themselves (wrong error code, mis-incremented counter, SQL column typo, etc.).

• catalog/migrate — MIGRATION_FAILED (db.exec spy) and MIGRATION_FK_VIOLATIONS (orphan FK row), confirming foreign_keys is re-enabled in finally in both cases
• dedupe/applier — FILE_MISSING guard: file in catalog but deleted from disk → failed=1, typed error message (not raw ENOENT)
• dedupe/scorer — zero-copy throw + single-copy 'only copy' branch
• quarantine — QUARANTINE_COLLISION, QUARANTINE_NOT_FOUND, QUARANTINE_FILE_MISSING typed errors with correct .code
• catalog/locator — defaultPointerPath/defaultCatalogPath with APPDATA set vs unset (env restored in afterEach)
• drives/volume — detectWindows happy path (execFileSync mocked via vi.mock + resolve spy for Linux CI) + subprocess-failure fallback to synthSerial
• scan/metadata-video — happy path via a real shell-script stub binary that prints canned MediaInfo JSON; exercises the full execFileAsync → parseMediainfoOutput pipeline
• catalog/batches-repo — updateOperationStatus with all optional fields (postHash, errorMessage, destPath); raw DB columns verified plus the OperationRecord mapping

Test plan

• All 8 clusters have at least the tests listed in the issue body
• Existing tests still pass: 296 engine baseline → 310; UI 21 unchanged; shared 8 unchanged
• Lint clean (npm run lint exits 0)
• Typecheck clean (npm run typecheck exits 0)
• No production code changes

Deviations

None. The volume.ts happy-path test revealed that detectWindows produces no structured warning log when execFileSync fails (it silently catches and falls back). The test verifies the fallback behavior is correct; the missing log is documented as a code comment in volume.test.ts but was not treated as a bug requiring a production fix — the fallback itself is safe.

Out of scope

• Full UI route test coverage (M8 added some; broader coverage is a separate issue)
• CLI formatStatus/formatBytes tests (Minor finding)
• reverseCompletedViaExisting (covered in M7)

Closes #30