synapse/handlers/sync.py _get_rooms_changed: newly_joined_rooms is List[str]
checked with `in` inside for-loop over all joined rooms (line 2252).
O(J*N) where J=joined rooms, N=newly joined. Fix: use Set. 99x at J=5000.
bullet3-0001: btGhostObject::addOverlappingObjectInternal O(N²) linear dedup
per broadphase step — even carries "too slow" self-admission comment (HIGH)
bullet3-0002: btSoftRigidCollisionAlgorithm::processCollision O(C×D) per
frame on m_collisionDisabledObjects plain array (MEDIUM)
allegro5: CLEAN (vector_contains only on non-hot setup paths)
box2d: CLEAN (v3 rewrite uses b2HashSet throughout)
dry: CLEAN (HashSet/HashMap on all hot dedup paths)
