DEFAULT_MAX_SUB_STEPS

Default cap on how many fixed steps PhysicsWorld will run in a single frame. Bounds the catch-up work after a long stall (a backgrounded tab, a GC pause) so the simulation never enters a "spiral of death" where each frame owes more steps than it can afford — past the cap, the leftover time is dropped and the simulation runs in slow motion for that one frame instead of freezing.