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/fx.gd

@@ -0,0 +1,51 @@
+extends Node
+## Fx — the one place gameplay code fires a visual effect. Code names WHICH effect and WHERE;
+## the .tscn owns HOW it looks (colour, curves, emission — all inspector-editable, no recompile).
+## Autoload singleton. Usage:  Fx.spawn(&"blood_burst", hit_transform)
+##
+## Scenes are loaded lazily (not preloaded) so this autoload compiles and boots even before the
+## placeholder .tscn are seeded — see fx/_seed_placeholders.gd.
+
+const _PATHS: Dictionary = {
+	&"blood_burst": "res://fx/blood_burst.tscn",
+	&"slam_dust": "res://fx/slam_dust.tscn",
+	&"spark": "res://fx/spark.tscn",
+	&"hit_flash": "res://fx/hit_flash.tscn",
+}
+
+var _cache: Dictionary = {}
+
+
+# Spawn a named effect at `xform`, parented to `parent` (or the current scene). Returns the
+# instance, or null if the effect is unknown or its scene is missing.
+func spawn(effect: StringName, xform: Transform3D, parent: Node = null) -> Node:
+	var scene := _scene(effect)
+	if scene == null:
+		return null
+	var host := parent if parent != null else _default_host()
+	if host == null:
+		push_warning("Fx.spawn(%s): no parent and no current scene to attach to" % effect)
+		return null
+	var inst := scene.instantiate() as Node3D
+	host.add_child(inst)
+	inst.global_transform = xform
+	return inst
+
+
+func _scene(effect: StringName) -> PackedScene:
+	if _cache.has(effect):
+		return _cache[effect]
+	if not _PATHS.has(effect):
+		push_warning("Fx.spawn: unknown effect '%s' (add it to fx/fx.gd)" % effect)
+		return null
+	var scene := load(_PATHS[effect]) as PackedScene
+	if scene == null:
+		push_warning("Fx.spawn: scene missing for '%s' at %s" % [effect, _PATHS[effect]])
+		return null
+	_cache[effect] = scene
+	return scene
+
+
+func _default_host() -> Node:
+	var tree := get_tree()
+	return tree.current_scene if tree != null else null