Backup classification: schema + parser + pure classifier (INERT, v0.132.0)

Task 2 of the backup-classification-redesign arc. Ships the referential-
coupling classification as DATA + PARSER + PURE CLASSIFIER, deliberately
inert — no backup tier changes behavior. Task 3 (tier policy engine) and
Task 4 (manual .fab UI) consume it.

- appbackup/classify.go: BackupSpec/BindSpec/ComposeBind/ClassifiedBind;
  ClassifyBinds (SQ5 two-level default — explicit beats :ro; unlisted
  writable→mandatory, unlisted :ro→excluded; nil spec→legacy/false);
  ValidateBackupSpec (whole-block-reject on any defect, first defect named).
- stacks/classify_binds.go: ParseComposeClassifiableBinds — ${VAR}-relative
  binds + :ro flag (NOT ParseComposeHDDMounts/ExportDataMounts, the traps).
- Metadata.Backup + LoadMetadata as the single validation choke point (bad
  catalog block → nil + one ERROR → legacy, within one sync cycle).
- Manager.ClassifiedBinds + StackDataProvider.GetStackClassifiedBinds seam
  (delegated by stackAdapter, nil-stubbed in every fake) — wired + tested
  now so Task 3 consumes a tested seam.

INERT: full pre-existing suite green with zero test-logic edits. +14 tests;
red-proofs RP-1..RP-4 confirmed. The 13 catalog backup: blocks ship in the
same app-catalog change (this controller deploys first).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A45Qop8YY8tS94bz63LFne

controller/internal/api/backup_snapshots_test.go

@@ -34,6 +34,9 @@ func (p *snapshotsStubProvider) RefreshAndIsRunning(string) bool           { ret
 func (p *snapshotsStubProvider) GetStackRecoveryInfo(string) (backup.RecoveryInfo, bool) {
 	return backup.RecoveryInfo{}, false
 }
+func (p *snapshotsStubProvider) GetStackClassifiedBinds(string) ([]backup.ClassifiedBind, bool) {
+	return nil, false
+}
 func (p *snapshotsStubProvider) RecoverStackSecrets(string, []string) map[string]string { return nil }
 func (p *snapshotsStubProvider) RecreateStackFromUnit(string, string, map[string]string) error {
 	return nil