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/hit_flash.tscn

@@ -0,0 +1,41 @@
+[gd_scene format=3]
+
+[ext_resource type="Script" path="res://fx/one_shot_effect.gd" id="1_6wb3q"]
+
+[sub_resource type="StandardMaterial3D" id="StandardMaterial3D_n65j7"]
+transparency = 1
+cull_mode = 2
+shading_mode = 0
+vertex_color_use_as_albedo = true
+
+[sub_resource type="SphereMesh" id="SphereMesh_ih2se"]
+material = SubResource("StandardMaterial3D_n65j7")
+radius = 0.05
+height = 0.1
+radial_segments = 4
+rings = 2
+
+[sub_resource type="Curve" id="Curve_kfjqk"]
+_data = [Vector2(0, 0.2), 0.0, 0.0, 0, 0, Vector2(0.3, 1), 0.0, 0.0, 0, 0, Vector2(1, 0), 0.0, 0.0, 0, 0]
+point_count = 3
+
+[sub_resource type="Gradient" id="Gradient_8o2om"]
+colors = PackedColorArray(1, 1, 1, 1, 1, 1, 1, 0)
+
+[node name="hit_flash" type="CPUParticles3D" unique_id=454164430]
+emitting = false
+amount = 6
+lifetime = 0.18
+one_shot = true
+explosiveness = 1.0
+randomness = 0.2
+mesh = SubResource("SphereMesh_ih2se")
+spread = 180.0
+gravity = Vector3(0, 0, 0)
+initial_velocity_min = 0.5
+initial_velocity_max = 1.5
+scale_amount_min = 0.8
+scale_amount_max = 1.4
+scale_amount_curve = SubResource("Curve_kfjqk")
+color_ramp = SubResource("Gradient_8o2om")
+script = ExtResource("1_6wb3q")