Lookup() and ListAll() now return value copies (not pointers to
internal registry state). Callers can no longer mutate registry
entries without holding a lock.
Added clone() on BlockVolumeEntry with deep-copied Replicas slice.
Added UpdateEntry(name, func(*BlockVolumeEntry)) for locked mutation.
ListByServer() also returns copies.
Migrated 1 production mutation (ReplicaPlacement + Preset in create
handler) and ~20 test mutations to use UpdateEntry.
5 new copy-correctness tests: Lookup returns copy, Replicas slice
isolated, ListAll returns copies, UpdateEntry mutates, UpdateEntry
not-found error.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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