Struct SubgraphLockBox 

pub struct SubgraphLockBox { /* private fields */ }

Per-component lock box. Holds the partition’s wave_owner re-entrant mutex. On union, the box of the smaller-rank root is replaced by the larger-rank root’s box — the lock identity is preserved across merges via the Arc reference (mirrors py’s _LockBox.lock redirect on union).

Slice Y1 (D3 / Phase E, 2026-05-08): the wave engine acquires this per-partition lock via crate::Core::partition_wave_owner_lock_arc (with retry-validate against concurrent union/split). Closure-form Core::batch acquires every partition’s lock in ascending SubgraphId order; per-seed entry points (emit, subscribe, etc.) acquire only the seed’s transitively-touched partitions.