seam sweep: compile-time witness for BackupArchiveLister (no version bump)

localapi.BackupArchiveLister is satisfied by a runtime type assertion in
newestArchiveOn; a failed assertion degrades SILENTLY to archiveAbsent, which is
the pre-R-84 in-memory-only behaviour — i.e. the R-84 bug returning with nothing
in any log to say so. There was no compile-time witness anywhere in production
code in either repo.

No defect found: *BackupRunner does satisfy it today, so this is a guard, not a
fix. Verified the guard works — breaking NewestArchiveTime's signature now fails
go build, where before it compiled and vetted clean.

No version bump, no deploy: compile-time only, zero runtime effect.

cmd/felhom-agent/seam_witnesses.go

@@ -0,0 +1,28 @@
+package main
+
+import (
+	"gitea.dooplex.hu/admin/felhom-agent/internal/backup"
+	"gitea.dooplex.hu/admin/felhom-agent/internal/localapi"
+)
+
+// COMPILE-TIME WITNESSES for OPTIONAL interfaces that are satisfied by a RUNTIME type assertion.
+//
+// WHY THIS FILE EXISTS. `localapi.BackupArchiveLister` is asserted at server.go's `newestArchiveOn`
+// via `tier.Service.(BackupArchiveLister)`. A failed assertion does not error — it degrades to
+// `archiveAbsent`, i.e. the pre-R-84 "ask the in-memory record only" behaviour. That degrade is
+// SILENT and it is behaviour-relevant: it is exactly the R-84 bug (a cold store after a restart
+// reading as "no backup ever") coming back, with nothing in any log to say so.
+//
+// The precedent is not hypothetical. During R-88 Part 2 the controller's `quiesceBackend` stopped
+// satisfying `quiesce.TieredBackend` when a signature changed, and `go build` AND `go vet` both
+// passed — because the interface is only ever asserted at runtime. Every box would have degraded to
+// the single-tier path, losing R-82's multi-tier backups, with no error anywhere. It was caught by
+// accident.
+//
+// A witness costs one line and converts that class of failure from a silent production degrade into
+// a compile error.
+//
+// THIS DOES NOT MAKE THE INTERFACE REQUIRED. The optionality is deliberate — it is what lets a
+// BackupService without a lister still work. The witness pins the IMPLEMENTATION (this concrete type
+// really does satisfy it), not the CONTRACT.
+var _ localapi.BackupArchiveLister = (*backup.BackupRunner)(nil)