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_candidates_test.go

@@ -0,0 +1,40 @@
+package localapi
+
+import (
+	"encoding/json"
+	"net/http"
+	"testing"
+
+	"gitea.dooplex.hu/admin/felhom-agent/internal/storage"
+)
+
+// handleDiskCandidates splits discovery into initialize (all unclaimed) + attach (mountable-FS subset).
+func TestDiskCandidates_Split(t *testing.T) {
+	d := &fakeDiskOps{candidates: []storage.CandidateDisk{
+		{Device: "/dev/sdd", SizeBytes: 64 << 30, DataBearing: false},                                    // blank → initialize only
+		{Device: "/dev/sde", FSType: "ext4", DataBearing: true, Mountable: true, MountSource: "/dev/sde1"}, // FS → init + attach
+		{Device: "/dev/sdf", FSType: "ntfs", DataBearing: true, Mountable: false},                         // ntfs → initialize only
+	}}
+	h := newDiskServer(t, d, &fakeGate{}, nil, nil)
+
+	w := do(t, h, "GET", "/disks/candidates", "A", "")
+	if w.Code != http.StatusOK {
+		t.Fatalf("candidates: got %d want 200 (%s)", w.Code, w.Body.String())
+	}
+	var env struct {
+		Data struct {
+			Initialize []storage.CandidateDisk `json:"initialize"`
+			Attach     []storage.CandidateDisk `json:"attach"`
+		} `json:"data"`
+	}
+	if err := json.Unmarshal(w.Body.Bytes(), &env); err != nil {
+		t.Fatalf("parse: %v", err)
+	}
+	resp := env.Data
+	if len(resp.Initialize) != 3 {
+		t.Fatalf("initialize must list all 3 unclaimed disks, got %d", len(resp.Initialize))
+	}
+	if len(resp.Attach) != 1 || resp.Attach[0].Device != "/dev/sde" {
+		t.Fatalf("attach must list only the mountable-FS disk, got %+v", resp.Attach)
+	}
+}