Add scene-authored FX effect library and Fx autoload

Replace code-buried CPUParticles with inspector-editable effect scenes fired
through one Fx autoload: code names the effect (Fx.spawn(&"blood_burst", xform)),
the .tscn owns the look. Ships four placeholder effects (blood_burst, slam_dust,
spark, hit_flash) seeded from the old code params, a self-freeing one_shot_effect
root helper, a live preview gallery, and an FX style guide for Siim. CPUParticles
only (mobile GLES3 constraint). Existing call sites untouched -- migrated later.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

fx/effect_gallery.gd

@@ -0,0 +1,30 @@
+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"]
+
+@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)))