# js/physics/step/ — agents.md
RC-375D84 S-2. Module map: `MODULES.md`; runtime model: `architecture.md`.

Installs into `P`: step, stepPhysics, spendBird. **The physics hub.**

- `step(g, dt)` = time advance + stepPhysics + end-state check + camera. `stepPhysics`
  is the integration core: gravity, air drag, bird/bird-block/bird-enemy/
  block-block contacts, crush, settle bookkeeping.
- Because it reaches into nearly every unit, it is installed AFTER aim/damage and
  BEFORE nothing — but all of its callees must exist in `P` by the time `step`
  runs (they do, because install order is load order). Keep it that way.
- `spendBird` retires a bird that is GONE (aged out / off the island). S-5
  (RC-375D84, landed 2026-09-29) split the settle branch: a bird that settles
  while ANOTHER bird is in play becomes `landed` — a live body integrated
  every frame in `stepPhysics` (real gravity, no lifetime timer) that falls
  when its support breaks, chain-damages boards, and can kill enemies. The LAST
  bird still spends (win/lose unchanged — keep it that way; the sim win/lose
  test is the gate). Landed birds are cleared when the level ends (inside
  `step`'s won/lost branch). `advanceToNextBird` is the shared "next bird onto
  the sling" path used by both the spent and landed transitions.
- Determinism invariant: no Math.random, no Date, no DOM. Tests call this
  directly at dt=1/120.