useFinalizeForIMOBounds

When true, IMOs are created with hostOutput=null during reduce (skipping per-child ensureHostBounds), and host bounds are finalized in a single bottom-up pass after reduce. Eliminates N recursive ensureHostBounds calls, replacing with N flat comparisons.

This is the umbrella flag for moving IMO position-dependent state out of construction and into LithoReducer.finalizeIncrementalMountBounds; subsequent diffs in the subtree cache Phase 1A series add additional finalize-driven passes (bounds initialization, hostOutput re-derivation) under this same gate.