agent v0.87.0: SystemDisks device-mapper walk — legacy-boot hosts get a working drive wizard (IA finding 2, MEDIUM)

Operator ruling 2026-07-13: walk the root's backing device through /sys/block/<dev>/slaves
recursively down to physical disks (dm AND md; topology, never VG names); those + any mounted-ESP
holder are system; the all-system fail-safe returns to being the WALK-FAILURE error case only.
SAFETY DIRECTION: a root-backing disk classified candidate is made impossible — per-branch
conservatism (any unresolvable slave fails the WHOLE walk -> ok=false -> the unchanged all-system
path).

- physicalDisksOf/walkSlaves in role.go (symlink canon -> wholeDiskOf fast path -> recursive
  slaves walk; cycle/depth guard; non-/dev sources unwalkable)
- HostReader.BlockSlaves(name) — the ONE new seam method; ProcHostReader reads
  /sys/block/<name>/slaves; all four test fakes mirror it
- role_walk_test.go: signature table (root-backing disk ALWAYS system across legacy-LVM /
  md-raid / EFI+raw / EFI+LVM / nested dm-on-md — NEVER weaken) + dead-wizard-lives +
  dangling-slave fail-safe (real sysKnown=false path) + cycle + empty-slaves; red-proofs A/B/D
  run->fail->revert (recorded in REPORT)
- §3 spike transcripts (drill legacy: dm-1->sda3->sda; felhom-pve: ESP+walk agree on sda ->
  byte-identical regression); caller audit: none relied on all-system as a feature
- format/mkfs paths, data-bearing guards, wizard UI untouched

internal/localapi/disks_test.go

@@ -90,11 +90,12 @@ func (f fakeGuestList) ListLXC(context.Context) ([]proxmox.Guest, error) { retur
 // a different whole-disk (e.g. /dev/sdb1) classifies as user-data without touching the real host.
 type fakeHostReader struct{ mounts []storage.Mount }
 
-func (f fakeHostReader) Mounts() ([]storage.Mount, error)  { return f.mounts, nil }
-func (f fakeHostReader) ResolveUUID(string) (string, bool) { return "", false }
-func (f fakeHostReader) DeviceExists(string) bool          { return true }
-func (f fakeHostReader) Rotational(string) (bool, bool)    { return false, false }
-func (f fakeHostReader) Removable(string) (bool, bool)     { return false, false }
+func (f fakeHostReader) Mounts() ([]storage.Mount, error)    { return f.mounts, nil }
+func (f fakeHostReader) ResolveUUID(string) (string, bool)   { return "", false }
+func (f fakeHostReader) DeviceExists(string) bool            { return true }
+func (f fakeHostReader) Rotational(string) (bool, bool)      { return false, false }
+func (f fakeHostReader) Removable(string) (bool, bool)       { return false, false }
+func (f fakeHostReader) BlockSlaves(string) ([]string, bool) { return nil, false }
 
 // sysOnSDA is the default system-disk fixture (root on /dev/sda) used by the disk-server test helpers.
 func sysOnSDA() fakeHostReader {