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>

tests/fx_gallery_test.gd

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+extends SceneTree
+## Smoke test for the FX library: every registered effect instantiates and attaches without error,
+## and an unknown name fails soft (returns null, no crash). Catches a broken effect .tscn before it
+## ships. Run: godot --headless --script tests/fx_gallery_test.gd
+
+var _pass := 0
+var _fail := 0
+
+
+func _init() -> void:
+	_run.call_deferred()
+
+
+func _ok(cond: bool, msg: String) -> void:
+	if cond:
+		_pass += 1
+		print("  PASS: ", msg)
+	else:
+		_fail += 1
+		print("  FAIL: ", msg)
+
+
+func _run() -> void:
+	var fx: Node = root.get_node("/root/Fx")
+	var host := Node3D.new()
+	root.add_child(host)
+	current_scene = host
+
+	for id: StringName in [&"blood_burst", &"slam_dust", &"spark", &"hit_flash"]:
+		var inst: Node = fx.call("spawn", id, Transform3D.IDENTITY, host)
+		_ok(inst != null and is_instance_valid(inst), "spawns %s" % id)
+
+	var missing: Node = fx.call("spawn", &"does_not_exist", Transform3D.IDENTITY, host)
+	_ok(missing == null, "unknown effect returns null instead of crashing")
+
+	# The preview gallery scene is wired (has its controller script + label).
+	var gallery: Node = load("res://fx/effect_gallery.tscn").instantiate()
+	_ok(gallery.get_script() != null, "gallery scene keeps its controller script")
+	host.add_child(gallery)
+	await process_frame
+	_ok(gallery.get_node_or_null("UI/Label") != null, "gallery scene has its name label")
+
+	await process_frame
+	print("Results: %d passed, %d failed" % [_pass, _fail])
+	quit(1 if _fail > 0 else 0)

tests/fx_gallery_test.gd.uid

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+uid://c2utxg1g3cpjs