R-116 (v0.116.0): give the backup-target flag and the gate's key the same row

The absent-drive alarm was generic while its recovery was specific -- a pair an
operator cannot match. Mechanism now measured, not reasoned (felhom.eu
audits/DIAG-r116-disks-payload-2026-07-30.md): with the device gone /disks returns
4 rows, not 3. The drive appears TWICE and the two facts the controller needs are
on different rows -- the Observe row has backup_target:true but mount_path:"" and
guest_path:"" (so driveTargetByPath registers NO key from it), while the registry
row owns /mnt/felhom-drives/<name>, the key the gate looks up, with BackupTarget
absent from its struct literal => false.

WHY v0.115.0 WAS INERT: its fallback computed StablePathForRaw(t.MountPath), and in
the absent state MountPath is ALSO "" -- emptied by the same exactMount failure
that empties BackingDevice. It assigned nothing. Its test passed because the fixture
supplied a MountPath production never supplies, and the harness left DriveTargets
nil so the union loop never ran. Both corrected here; red-proof 1 replays v0.115.0's
exact code against the real shape and it fails.

THE JOIN, which was the hard part: with the device gone the two records share no
runtime field -- no mount, no backing device, and the Observe row's DurableID has
degraded off the fs-UUID. They share CONFIGURATION: storage.cfg's path on one side,
the .mount unit's Where on the other, both yielding the same stable guest path. New
hub.StorageTarget.ConfigPath (json:"-" -- that struct is a cross-repo contract
pinned by the golden + contract_test key-set comparison, and nothing off-box needs
the value), set from s.Path in observe.go, consulted in disks.go only after MountPath
so the present-state path is byte-identical, plus a guest-path arm on the union dedup
so exactly one row carries the drive.

WHY NEITHER OBVIOUS OPTION WAS TAKEN -- both regress R-114, which shipped yesterday.
backup_target_offer.go:79 reads (BackupTarget && MountPath != "") as "a real
drive with its own mountpoint -- healthy" and returns before its TargetAbsent
branch. Back-filling MountPath onto the Observe row (the smallest change, and the
spec's lean) and teaching the registry row the flag (its MountPath is non-empty, read
from the stale unit file) BOTH manufacture that row while the drive is missing, which
would have told the customer the backup target is fine while its drive is gone.
R-114's correctness rests on the absent-state rows not combining the flag with a
mount path; that coupling was invisible until the payload existed. Pinned by
TestAbsentTargetKeepsR114DegradedSignal.

Role unchanged, BoundUnderParent conjunction not widened, no wire field changed.
Suppressing the registry row in the absent state also removes its false
state:"attached" and its root-filesystem-derived total_bytes -- R-118's symptom
goes incidentally; R-118 is NOT fixed and stays open.

Tests 845 -> 849, suite rc=0 read separately from this commit. Four red-proofs, each
mutation asserted to have landed first.

NOT live-validated at this commit: publish+vouch, C5, discrimination, over-correction.

internal/storage/observe.go

@@ -235,9 +235,16 @@ func (o *Observer) build(s proxmox.Storage, mounts []Mount) observed {
 		Content:       s.Content,
 		MountPath:     mountPath,
 		BackingDevice: backingDevice,
-		ClassHint:     classHint,
-		Role:          "", // hub-owned; not derivable from a Proxmox def (slice 10)
-		Smart:         hub.SmartSummary{Health: hub.SmartUnknown},
+		// R-116: the CONFIGURED path, carried verbatim and never resolved. This is emphatically NOT the
+		// fallthrough the comment above forbids — that prohibition is about resolving a device or a UUID
+		// from the CONTAINING filesystem when the target is not its own mount, which would hand back
+		// root's identity and mis-target a DR re-attach. `s.Path` is the storage's own declaration of
+		// where it lives; it identifies nothing but itself, and it is not used for device or UUID
+		// resolution anywhere. MountPath stays empty when the mount is gone, which is the truth.
+		ConfigPath: s.Path,
+		ClassHint:  classHint,
+		Role:       "", // hub-owned; not derivable from a Proxmox def (slice 10)
+		Smart:      hub.SmartSummary{Health: hub.SmartUnknown},
 	}
 
 	// Thin-pool DATA fill: surfaced prominently for lvmthin (metadata fill is Phase B/lvs).