Complete the DamageEvent migration: one deal() entry point for every attacker

The bull's Hitbox already routed through DamageEvent, but the spear, the bear's
maul, and the matador's reach-gore still did raw Object.call(&"take_sword_hit", ...)
— the exact scattered string paths DamageEvent was meant to retire. Add a single
static DamageEvent.deal(target, cause, at, source, knock_up) that builds the typed
event and hands it to receive_damage(), falling back to the legacy string call for
any not-yet-migrated victim, and route all four attackers (Hitbox included) through
it. The damage_pipeline characterization test proves the bull's outcomes are
unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Pq1VX2o1ZfiNRdCjD294wf

spear_projectile.gd

@@ -223,7 +223,7 @@ func _embed(other: Object, contact: Vector3, flight: Vector3) -> void:
 	var pose := Transform3D(_aim_basis(f), contact + f * (_EMBED_DEPTH - _TIP_Z))
 	var bull := other as Node
 	if bull != null and bull.is_in_group(&"player"):
-		bull.call(&"take_sword_hit", "thrown", contact)
+		DamageEvent.deal(bull, DamageEvent.Cause.THROWN, contact, self)
 		if is_instance_valid(_mesh) and is_instance_valid(bull):
 			_mesh.reparent(_spear_anchor(bull), false)
 			_mesh.global_transform = pose