whaleflow: janitor — stale invalidation, memo cleanup, demotion, trace compaction

[Hmbown]

Implement the janitor: stale invalidation, memo cleanup, candidate demotion, trace compaction, and capacity enforcement over TraceStore/memo/overlay state.

Acceptance: capacity limits enforced under test; compaction preserves replayability of retained traces.

Part of the v0.9.0 EPIC.