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. Find materializer with shard_id = 0 (system shard) in available_services
2. If not found, create a new materializer on a capable node
3. Lock materializer for recovery
4. Unlock it with system shard logs to start pulling
5. Wait for materializer to catch up (60s timeout)
6. Query shard layout from \xff/system/shard_keys/*

Stalls if the materializer is unavailable and cannot be created, or if catchup times out. Transitions to CommitProxyStartupPhase with the materializer pid and shard layout.