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/testdata/host-report.golden.json

@@ -46,6 +46,9 @@
       "backing_device": "",
       "class_hint": "fast",
       "role": "",
+      "kind": "system",
+      "physical_disks": ["serial:ata:SanDisk_X600:182195804614"],
+      "identity": "proven",
       "thin_pool": { "data_used_fraction": 0.42, "metadata_used_fraction": null },
       "smart": {
         "health": "UNKNOWN",
@@ -74,6 +77,10 @@
       "backing_device": "/dev/sdb1",
       "class_hint": "slow",
       "role": "",
+      "kind": "disk",
+      "physical_disks": ["serial:usb:External:000000000000"],
+      "identity": "unknown",
+      "identity_reason": "usb_bridge",
       "smart": {
         "health": "UNKNOWN",
         "temperature_c": null,