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/one_shot_effect.gd

@@ -0,0 +1,33 @@
+extends CPUParticles3D
+## Root helper for a one-shot effect scene: fires this emitter (and any CPUParticles3D children)
+## once when spawned, then frees the whole scene after the longest-lived particle has died. This
+## is what lets an effect be nothing but a tunable CPUParticles3D scene — no per-effect code, so
+## Siim owns the look entirely in the inspector. Fired by the Fx autoload; see fx/README.md.
+
+const _FREE_MARGIN := 0.5
+
+
+func _ready() -> void:
+	var ttl := _arm(self)
+	for emitter: CPUParticles3D in _child_emitters():
+		ttl = maxf(ttl, _arm(emitter))
+	get_tree().create_timer(ttl + _FREE_MARGIN).timeout.connect(queue_free)
+
+
+func _arm(p: CPUParticles3D) -> float:
+	p.one_shot = true
+	p.emitting = true
+	return p.lifetime
+
+
+func _child_emitters() -> Array[CPUParticles3D]:
+	var out: Array[CPUParticles3D] = []
+	_gather(self, out)
+	return out
+
+
+func _gather(node: Node, out: Array[CPUParticles3D]) -> void:
+	for child: Node in node.get_children():
+		if child is CPUParticles3D:
+			out.append(child)
+		_gather(child, out)