R-85 Phase 4: finalise v0.104.0 + register the new helpers in REUSE

Version finalised from v0.104.0-dev — an ldflags version disagreeing with the
CHANGELOG is the reconciliation problem hub 0.73.2 already caused.

REUSE gains backup.InFlight, backup.RestoreTestState and the SpecBuilder/
TierPicker/PickRestoreCandidateOn trio, each with the decision someone could
unknowingly undo: rotation credit only on success; InFlight callers defer and
never cancel; a tier with no archive no-ops rather than failing; SourceTier
comes from the archive, never the configured target.

CHANGELOG.md

@@ -1,4 +1,4 @@
-## v0.104.0-dev — R-85 Phase 2: tier rotation, persisted state, one heavy op at a time (2026-07-26)
+## v0.104.0 — R-85: unattended per-tier restore-test (Phase 2): tier rotation, persisted state, one heavy op at a time (2026-07-26)
 
 The scheduler could only ever see `cfg.Backup.BackupTarget()`, so the offsite tier's archives were
 never candidates. That is why demo-hp's DR tier reported `applied` with **zero snapshots for five
@@ -46,7 +46,7 @@ otherwise a permanently broken tier would look freshly proven and quietly stop b
 
 Full suite green (29 packages, `rc=0`, vet unpiped).
 
-## v0.104.0-dev — R-85 Phase 1: the restore-test spec is built PER RUN (2026-07-26)
+## v0.104.0 (cont.) — R-85 Phase 1: the restore-test spec is built PER RUN (2026-07-26)
 
 Prerequisite for scheduling the offsite tier at all. Shipped on its own because it is independently
 correct and independently testable.

REUSE.md

@@ -144,6 +144,9 @@
 | `storage.HostReader` | internal/storage/hostread.go | `*ProcHostReader` | `fakeHostReader` internal/localapi/disks_test.go; internal/storage/role_test.go. v0.87.0: `BlockSlaves(name)` lists `/sys/block/<name>/slaves` (root-free) — backs the `SystemDisks` dm/md walk (`physicalDisksOf`/`walkSlaves`, role.go); per-branch conservatism: an unresolvable slave fails the WHOLE walk → all-system fail-safe. NEVER weaken the signature test `TestSystemDisks_WalkTopologies` (root-backing disk always in the system set). |
 | `localapi.DiskOps` / `StorageGate` / `GuestAttacher` / `GuestLister` | internal/localapi/disks.go | `*storage.SudoHostOps`; `storageGateAdapter` (cmd/felhom-agent/main.go); `*GuestBinder`; `*proxmox.Client` | `fakeDiskOps`/`fakeGate`/`fakeGuestAttacher`/`fakeGuestList` internal/localapi/disks_test.go |
 | `localapi.GuestAPI` / `BackupService` / `BackupStore` / `TokenAuthority` | internal/localapi/server.go | `*proxmox.Client`, `*backup.BackupRunner`, `*backup.Store`, `*TokenStore` | `fakeGuests`/`fakeBackups`/`fakeStore` internal/localapi/server_test.go |
+| `backup.InFlight` | internal/backup/inflight.go | `TryAcquire(what) (release, busy, ok)` / `Busy()` | THE host-wide "one heavy guest operation at a time" gate — shared by the local-API backup path and the restore-test scheduler (R-85) | A **LINK** guard, not a lock one: the scratch VMID never touches the live guest's vzdump lock, but an offsite restore PULLS multi-GB over the tunnel a backup PUSHES one. Callers **DEFER, never cancel** — a deferred restore-test costs coverage, a cancelled backup costs the backup. A nil gate is ungated (pre-R-85 callers). |
+| `backup.RestoreTestState` | internal/backup/restoretest_state.go | `RecordSuccess(target,t)` / `LastSuccess(target)` / `OldestFirst(targets)` | Per-tier restore-test rotation state, persisted (atomic tmp+rename) | **Credit ONLY on success** — a permanently failing tier must keep sorting first, or it looks freshly proven and stops being retried. Ties break on target id: without it, two tiers proven in the same second rotate by Go's randomised map order. **This one NEEDS persistence unlike R-84** — R-84 had ground truth to consult (the archive is still on the storage); a restore-test destroys its scratch and leaves no artifact. |
+| `backup.SpecBuilder` / `backup.TierPicker` / `(*BackupRunner).PickRestoreCandidateOn` | internal/backup/schedule.go, runner.go | `func(ctx,archive) RestoreTestSpec`; `func(ctx,target) (string,error)` | The per-run restore-test spec + per-tier candidate lookup (R-85) | The spec is built **PER RUN**, never frozen at construction — the pre-R-85 immediately-invoked value made the offsite tier unschedulable AND went stale on any config change. `SourceTier` comes from **the archive**, never the configured target (the v0.100.0 rule). A tier with no archive returns `("", nil)` — **`""` is NOT an error**, or every fresh box looks broken for its first week. |
 | `localapi.BackupTier` + `normalizeBackupTiers` / `config.BackupConfig.BackupTiers` | internal/localapi/backup_tiers.go, internal/config/config.go | `normalizeBackupTiers(tiers, legacy, cadence) []BackupTier`; `BackupTiers() ([]BackupTier, []string)` | THE R-82 multi-tier resolution — one runner per tier, primary first | **The untargeted local-API contract is FROZEN**: no `?target=` ⇒ primary tier ⇒ pre-R-82 response BYTES (Target is `omitempty` and stays empty). Never default a missing cadence — reject it and log the warning at ERROR. Never share one retention knob between tiers. Jobs are keyed by (vmid,target). |
 | `localapi.StaleLockController` | internal/localapi/stalelock.go | `*staleLockController` (Client + Runner + pool) | `fakeStaleLock` (Server-level) stalelock_test.go; `fakeStaleLockAPI` (controller-level, tests the A1 pool intersect) stalelock_pool_test.go |
 | `localapi.GuestExecutor` | internal/localapi/controllerswap.go | `*GuestBinder` (pct exec) | `fakeGuestExec` internal/localapi/controllerswap_test.go |