Add scene-authored FX library and bull ability effects

An Fx autoload + one_shot_effect base so every effect is an inspector-editable
.tscn (the systems/fx/ authoring dir): blood/slam/spark/flash primitives and the
bull's kick/dash/slam/roll + movement effects (bull_*.tscn), fired via
Fx.spawn() / Fx.attach(). Plus a preview gallery + smoke test, and
bull_impact_fx.gd for the coded, non-particle slam rings + kick flash-pop.

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

systems/fx/effect_gallery.gd

@@ -0,0 +1,33 @@
+extends Node3D
+## Live preview stage for the FX library. Cycles through every registered effect on a loop against
+## arena-ish lighting so Siim sees each one as it reads in-game. Open fx/effect_gallery.tscn and
+## press Play, then edit any effect .tscn in the inspector and re-run to compare. See fx/README.md.
+
+const _CYCLE_SEC := 1.6
+const _EFFECTS: Array[StringName] = [
+	&"blood_burst", &"slam_dust", &"spark", &"hit_flash",
+	&"bull_slam", &"bull_kick", &"bull_dash",
+]
+
+@onready var _label: Label = $UI/Label
+# Reached by path rather than the Fx global so the placeholder-seeder can compile and attach this
+# script headless (autoload globals aren't resolvable during an on-demand compile in a --script run).
+@onready var _fx: Node = get_node("/root/Fx")
+
+var _index := 0
+
+
+func _ready() -> void:
+	var timer := Timer.new()
+	timer.wait_time = _CYCLE_SEC
+	timer.autostart = true
+	add_child(timer)
+	timer.timeout.connect(_fire_next)
+	_fire_next()
+
+
+func _fire_next() -> void:
+	var effect := _EFFECTS[_index % _EFFECTS.size()]
+	_index += 1
+	_label.text = "%s   (%d/%d)" % [effect, (_index - 1) % _EFFECTS.size() + 1, _EFFECTS.size()]
+	_fx.call("spawn", effect, global_transform.translated_local(Vector3(0.0, 0.25, 0.0)))