Add persist-backed compute introspection sources

[antiguru]

Reintroduces direct persist writes from compute replicas for introspection data, bypassing environmentd for the data path.
The coordinator's role is limited to shard allocation and lifecycle management.

• Per-replica persist shards for all ~30 log variants via the MV persist sink
• CREATE CLUSTER ... (PERSIST INTROSPECTION = true) opt-in + enable_persist_introspection dyncfg kill switch
• LoggingConfig.sink_logs carries shard metadata to replicas
• Deterministic GlobalId::PersistedIntrospectionSource encoding per replica+variant
• Shard allocation on replica create, wired through coordinator → controller → compute
• mzcompose integration tests for syntax, dyncfg, lifecycle, restart, catalog changes

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[github-actions]

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