notify_gateway_scene() + hints puzzle_start lived only in
npc_engine_set_step(), which had NO callers — the runtime->gateway
sync was dead code, so scripted incoming calls would never fire in a
real game. POST /game/step goes through game_endpoint_apply_step(),
which now resolves the step's scene_id (new field on scene_binding,
parsed straight from the IR) and calls the new
npc_engine_set_scene_by_id(). Hardware-validated: STEP_WARNING ->
scene 3 -> gateway notify -> auto-ring Professeur Zacus on the PLIP.
clement