Add in-memory type index for query_by_type_prefix

[iamnbutler]

Summary

• Adds a lazily-initialized in-memory secondary index (HashMap<event_type, Vec<Event>>) to EventStore that eliminates the O(n*m) full-scan in query_by_type_prefix()
• The index is built from disk on first query, then kept in sync on every append() call — subsequent queries only iterate over matching type keys
• Compaction and orphan cleanup invalidate the index so it rebuilds on next query
• Adds two new tests: index sync after append, and index rebuild after compaction

Closes #507

Test plan

• All 55 existing + new unit tests pass (cargo test --package events)
• Full project builds clean (cargo check)
• Verify orchestrator and API endpoints still work correctly with the indexed path

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