Add typed DamageEvent hit pipeline

A typed DamageEvent contract (systems/combat/damage_event.gd) as a single
entry point for every attacker, plus a characterization test locking the bull's
damage reception. Attacker/receiver wiring rides along with the moved actor
scripts in the reorg commit.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CCyaJPTHCH9EDthpf4A5Sj

tests/damage_pipeline_test.gd

@@ -0,0 +1,113 @@
+extends SceneTree
+## Characterization test for the bull's damage reception. It pins the OBSERVABLE behaviour of
+## take_sword_hit / apply_knock_up — pip loss, i-frame absorption, a single tagged death, and
+## upward-only knock-up — before the WS2 slice reroutes them through DamageEvent, so the rewire can
+## prove it changed nothing. When receive_damage(DamageEvent) lands, add a block that drives it and
+## asserts the identical outcomes.
+## Run: godot --headless --script tests/damage_pipeline_test.gd
+
+var _pass := 0
+var _fail := 0
+var _died_count := 0
+var _last_cause := ""
+var _last_hp := -1
+
+
+func _init() -> void:
+	_run.call_deferred()
+
+
+func _ok(cond: bool, msg: String) -> void:
+	if cond:
+		_pass += 1
+		print("  PASS: ", msg)
+	else:
+		_fail += 1
+		print("  FAIL: ", msg)
+
+
+func _on_died(cause: String) -> void:
+	_died_count += 1
+	_last_cause = cause
+
+
+func _on_health(current: int, _max_hp: int) -> void:
+	_last_hp = current
+
+
+func _run() -> void:
+	(root.get_node("/root/Run") as Node).call("ensure_run")
+	var scene: Node = load("res://levels/game_shell.tscn").instantiate()
+	root.add_child(scene)
+	current_scene = scene
+	await create_timer(0.8).timeout
+
+	var players := get_nodes_in_group(&"player")
+	_ok(not players.is_empty(), "bull present in the shell")
+	if players.is_empty():
+		return _finish()
+	var bull: Node = players[0]
+	bull.connect(&"died", _on_died)
+	bull.connect(&"health_changed", _on_health)
+
+	# Everything below runs synchronously (no await), so no physics frame or enemy turn interleaves —
+	# the bull's state changes only from the calls under test.
+	bull.set(&"_max_hp", 3)
+	bull.set(&"_hp", 3)
+	bull.set(&"_dead", false)
+	bull.set(&"_hit_iframes", 0.0)
+	bull.set(&"velocity", Vector3.ZERO)
+
+	bull.call(&"take_sword_hit", "gored")
+	_ok(int(bull.get(&"_hp")) == 2, "a clean hit removes one pip (3→2)")
+	_ok(_last_hp == 2, "health_changed reports the new pip count")
+
+	bull.call(&"take_sword_hit", "gored")
+	_ok(int(bull.get(&"_hp")) == 2, "i-frames absorb a second hit in the same window")
+
+	bull.set(&"_hit_iframes", 0.0)
+	bull.call(&"take_sword_hit", "gored")
+	_ok(int(bull.get(&"_hp")) == 1, "a hit after i-frames removes another pip (2→1)")
+
+	bull.set(&"_hit_iframes", 0.0)
+	bull.call(&"take_sword_hit", "gored")
+	_ok(int(bull.get(&"_hp")) <= 0, "the last pip drops the bull")
+	_ok(_died_count == 1, "died emits exactly once")
+	_ok(_last_cause == "gored", "died carries the hit's cause")
+
+	bull.set(&"_hit_iframes", 0.0)
+	bull.call(&"take_sword_hit", "thrown")
+	_ok(_died_count == 1, "a downed bull takes no further hits (death latches)")
+
+	bull.set(&"velocity", Vector3.ZERO)
+	bull.call(&"apply_knock_up", 5.0)
+	_ok(is_equal_approx(float((bull.get(&"velocity") as Vector3).y), 5.0), "knock_up raises velocity.y to the pop")
+	bull.call(&"apply_knock_up", 3.0)
+	_ok(is_equal_approx(float((bull.get(&"velocity") as Vector3).y), 5.0), "a smaller knock_up never lowers it")
+	bull.call(&"apply_knock_up", 8.0)
+	_ok(is_equal_approx(float((bull.get(&"velocity") as Vector3).y), 8.0), "a larger knock_up raises it further")
+
+	# The typed pipeline (Hitbox → DamageEvent → receive_damage) must produce identical outcomes to
+	# the string path it now wraps.
+	_last_hp = -1
+	bull.set(&"_hp", 3)
+	bull.set(&"_dead", false)
+	bull.set(&"_hit_iframes", 0.0)
+	bull.set(&"velocity", Vector3.ZERO)
+	bull.call(&"receive_damage", DamageEvent.make(DamageEvent.Cause.MAULED, Vector3.ZERO))
+	_ok(int(bull.get(&"_hp")) == 2, "receive_damage removes a pip like take_sword_hit")
+	_ok(_last_hp == 2, "receive_damage emits health_changed with the new pip count")
+
+	bull.set(&"_hit_iframes", 0.0)
+	bull.set(&"velocity", Vector3.ZERO)
+	var popping := DamageEvent.make(DamageEvent.Cause.SLAMMED, Vector3.ZERO)
+	popping.knock_up = 6.0
+	bull.call(&"receive_damage", popping)
+	_ok(is_equal_approx(float((bull.get(&"velocity") as Vector3).y), 6.0), "receive_damage applies the event's knock_up")
+
+	_finish()
+
+
+func _finish() -> void:
+	print("Results: %d passed, %d failed" % [_pass, _fail])
+	quit(1 if _fail > 0 else 0)