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/slam_dust.tscn

@@ -0,0 +1,44 @@
+[gd_scene format=3]
+
+[ext_resource type="Script" path="res://fx/one_shot_effect.gd" id="1_88uyl"]
+
+[sub_resource type="StandardMaterial3D" id="StandardMaterial3D_1tr47"]
+transparency = 1
+cull_mode = 2
+shading_mode = 0
+vertex_color_use_as_albedo = true
+
+[sub_resource type="SphereMesh" id="SphereMesh_bw8nk"]
+material = SubResource("StandardMaterial3D_1tr47")
+radius = 0.16
+height = 0.32
+radial_segments = 4
+rings = 2
+
+[sub_resource type="Curve" id="Curve_xgm72"]
+_data = [Vector2(0, 0.4), 0.0, 0.0, 0, 0, Vector2(0.35, 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_l8m5g"]
+colors = PackedColorArray(0.72, 0.6, 0.4, 0.9, 0.55, 0.45, 0.3, 0)
+
+[node name="slam_dust" type="CPUParticles3D" unique_id=82833428]
+emitting = false
+amount = 36
+lifetime = 0.9
+one_shot = true
+explosiveness = 1.0
+randomness = 0.5
+mesh = SubResource("SphereMesh_bw8nk")
+emission_shape = 1
+emission_sphere_radius = 0.5
+direction = Vector3(0, 1, 0)
+spread = 75.0
+gravity = Vector3(0, -6, 0)
+initial_velocity_min = 2.5
+initial_velocity_max = 6.5
+scale_amount_min = 0.6
+scale_amount_max = 1.4
+scale_amount_curve = SubResource("Curve_xgm72")
+color_ramp = SubResource("Gradient_l8m5g")
+script = ExtResource("1_88uyl")