Bootstraps the metadata shard materializer for recovery.

The metadata materializer holds the authoritative shard layout - the mapping from key ranges to shard tags. This phase ensures the materializer is available and queries it for the current shard layout.

Fresh Cluster

For a fresh cluster (no old logs), creates a default shard layout with two shards:

• System shard (tag 0): Keys from 0xFF to end-of-keyspace (system metadata)
• User shard (tag 1): Keys from empty string to 0xFF (user data)

Existing Cluster

For an existing cluster:

1. Resolve the system materializer BY NAME from the prior core state — it is looked up, never invented (bedrock-q67.21.12)
2. Lock it for recovery
3. Unlock it with its replica set of pull sources to start pulling
4. Wait for it to catch up (60s timeout)
5. Query the shard layout from \xff/system/shard_keys/*

Stalls if the named members are unavailable, if catchup times out, or if the recovered layout reads empty. Records written before the core state carried system materializers take a one-time migration: recovery adopts the sole locked worker claiming tag 0, and refuses to choose when several do (bedrock-q67.21.21).

Transitions to CommitProxyStartupPhase with the materializer pid and shard layout.