[eno][backup 5/7] Record synthesis inventory

[Ruinan Liu (ruinan-liu)]

Summary

Complete the opt-in backup lifecycle by recording inventory after recovery finishes and the current synthesis becomes ready. This PR is stacked on #655.

Behavior

• Build the current synthesis inventory from its complete ResourceSlice set after readiness.
• Create and verify the synthesis-specific ConfigMap in downstream kube-system.
• Preserve historical inventory when recovery recorded an unresolved inventory read or invalid inventory.
• Delete other inventories in the same namespace-and-synthesizer lineage only after the replacement is persisted and verified.
• Keep inventory recording status-free; it does not modify Composition status or repeat the recovery diff.

Race conditions

• Re-read the Composition directly before snapshot creation and before each old-inventory deletion, requiring the same current synthesis UUID, readiness, and nondeleting state.
• Include the synthesis UUID in the snapshot name so obsolete work cannot overwrite a newer synthesis snapshot.
• On AlreadyExists, verify the stored snapshot exactly instead of treating existence as success or overwriting it.
• Delete only inventories returned by the operation's initial lineage-scoped list, with UID and resourceVersion preconditions to protect replaced or modified ConfigMaps.

These checks reject obsolete operations but are not a cross-cluster transaction; a synthesis may still change immediately after a freshness read.

Stack

1. Recovery API and helper foundation.
2. [eno][tombstone-recovery 4/7] Recover missing tombstones #655: tombstone recovery controller and gate.
3. This PR: ready-synthesis inventory recording and rotation.

Inventory retirement and periodic garbage collection remain follow-up work. End-to-end coverage for backup and tombstone recovery will be added in a later effort.