v0.55.0: raw-device discovery + registry-sourced drive tracking (Impl-2a)

GET /disks/candidates enumerates host disks the Impl-1 unclaimed filter proves
free (init/attach split). RegistryKnownTargets sources the watchdog's known-drive
set from the intent registry + Felhom .mount units (not Observe/PVE storages) —
decouples drive health from PVE storage (closes the registry-only false-detach
class); Observe kept for real PVE storages + a deduped /disks union. Idempotent
existing-drive migration at start. Tests + red-proof (Observe misses a
registry-only drive; registry provider tracks it). go build/vet/test clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/localapi/disks_test.go

@@ -26,6 +26,12 @@ type fakeDiskOps struct {
 	formatCalls  []string
 	mountCalls   []storage.MountSpec
 	unmountCalls []string
+	candidates   []storage.CandidateDisk // returned by ListCandidateDisks
+	candErr      error
+}
+
+func (f *fakeDiskOps) ListCandidateDisks(_ context.Context) ([]storage.CandidateDisk, error) {
+	return f.candidates, f.candErr
 }
 
 func (f *fakeDiskOps) InspectDevice(_ context.Context, device string) (storage.DeviceProbe, error) {