Physical-disk identity of every storage place + POST /disks/same-disk (felhom.eu architecture/16 §3/§4)

The agent names the physical disk(s) under each storage place from /sys, udev and /proc/mounts only
(partition -> disk, dm/LVM thin -> slaves, loop -> backing file), keys them (eui / serial:<bus>:<model>
:<serial> / vendor WWN) and says proven or unknown (vm, network, usb_bridge, weak_wwn, no_serial).
GET /disks and the hub report's storage_targets gain kind, physical_disks, identity,
identity_reason (golden updated; the hub copy changes in the same session). POST /disks/same-disk
maps two guest paths to places the agent itself made and answers {different, proven, reason};
an absent drive is unmapped, never the root disk. Tests on fixture trees for every spike case,
red-proofed. CHANGELOG entry follows the release (release-complete gate).

Claude-Session: https://claude.ai/code/session_012qRErfCoiTkvDK9N5XHbzb

internal/hub/contract_test.go

@@ -39,11 +39,14 @@ func TestHostReport_ContractMatchesGolden(t *testing.T) {
 				State: StorageStateAttached, Reachable: true, ClassHint: "fast",
 				ThinPool: &ThinPoolFill{DataUsedFraction: 0.42},
 				Smart:    SmartSummary{Health: SmartUnknown},
+				// v0.157.0 (`16` §3): the identity keys ride on both rows so the key-set guard sees them.
+				Kind: "system", PhysicalDisks: []string{"serial:ata:SanDisk_X600:182195804614"}, Identity: "proven",
 			},
 			{
 				Name: "usb-backup", Type: StorageTypeUSB, DurableID: "uuid:x",
 				State: StorageStateAttached, Reachable: true, MountPath: "/mnt/usb-backup", TotalBytes: 2000000000000,
 				Smart: SmartSummary{Health: SmartUnknown},
+				Kind:  "disk", PhysicalDisks: []string{"serial:usb:External:000000000000"}, Identity: "unknown", IdentityReason: "usb_bridge",
 			},
 			// A pbs target so the recipe's pbs coord has a storage.cfg row to resolve its namespace
 			// from (R-106). Without one the fixture produces namespace_state=unknown while the golden