Split out of WT-18644. Shared metadata queue tracing is interleaved with all other disaggregated storage messages under WT_VERB_DISAGGREGATED_STORAGE.

• Add WT_VERB_SCHEMA_EPOCH (schema_epoch).
• Move the queue lifecycle messages in conn_layered.c to it: enqueue, cancel, apply, CREATE/REMOVE pairing, epoch deferral, parked creates, deferred flag, publish, and the epoch-mode step-up stable table creation.
• Unchanged: legacy step-up rebuild, step-up summary, drop size and checkpoint start stay in disaggregated_storage; schema epoch updates in txn_timestamp.c stay in timestamp.

Note: mongod controls only the WiredTiger categories it maps (wiredtiger_util.cpp). A SERVER follow-up is needed before the category can be set from mongod.