Add scene-authored FX library and bull ability effects

An Fx autoload + one_shot_effect base so every effect is an inspector-editable
.tscn (the systems/fx/ authoring dir): blood/slam/spark/flash primitives and the
bull's kick/dash/slam/roll + movement effects (bull_*.tscn), fired via
Fx.spawn() / Fx.attach(). Plus a preview gallery + smoke test, and
bull_impact_fx.gd for the coded, non-particle slam rings + kick flash-pop.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CCyaJPTHCH9EDthpf4A5Sj

systems/fx/bull_kick.tscn

@@ -0,0 +1,115 @@
+[gd_scene format=3]
+
+; Bull back-kick impact — the one-shot combined dust + stones + sparks thrown behind the kick.
+; A composite effect: the root emitter is the dust; the "stones" and "sparks" child emitters fire
+; with it (one_shot_effect.gd arms every CPUParticles3D in the scene). Fired via
+; Fx.spawn(&"bull_kick", xform); the spawn transform aims local -Z along the kick-back direction.
+; The bright flash-pop is a separate coded effect (bull_impact_fx.gd). Tune all three looks here.
+
+[ext_resource type="Script" path="res://systems/fx/one_shot_effect.gd" id="oneshot"]
+
+[sub_resource type="StandardMaterial3D" id="mat"]
+transparency = 1
+cull_mode = 2
+shading_mode = 0
+vertex_color_use_as_albedo = true
+
+[sub_resource type="SphereMesh" id="dust_mesh"]
+material = SubResource("mat")
+radius = 0.12
+height = 0.24
+radial_segments = 4
+rings = 2
+
+[sub_resource type="Curve" id="dust_curve"]
+_data = [Vector2(0, 0.3), 0.0, 0.0, 0, 0, Vector2(0.3, 1.5), 0.0, 0.0, 0, 0, Vector2(1, 0), 0.0, 0.0, 0, 0]
+point_count = 3
+
+[sub_resource type="Gradient" id="dust_grad"]
+colors = PackedColorArray(1, 0.78, 0.18, 0.95, 1, 0.45, 0.05, 0)
+
+[sub_resource type="SphereMesh" id="stone_mesh"]
+material = SubResource("mat")
+radius = 0.055
+height = 0.11
+radial_segments = 3
+rings = 1
+
+[sub_resource type="Curve" id="stone_curve"]
+_data = [Vector2(0, 1), 0.0, 0.0, 0, 0, Vector2(0.6, 1), 0.0, 0.0, 0, 0, Vector2(1, 0), 0.0, 0.0, 0, 0]
+point_count = 3
+
+[sub_resource type="Gradient" id="stone_grad"]
+colors = PackedColorArray(0.62, 0.58, 0.52, 1, 0.62, 0.58, 0.52, 0)
+
+[sub_resource type="SphereMesh" id="spark_mesh"]
+material = SubResource("mat")
+radius = 0.04
+height = 0.08
+radial_segments = 3
+rings = 1
+
+[sub_resource type="Curve" id="spark_curve"]
+_data = [Vector2(0, 1), 0.0, 0.0, 0, 0, Vector2(0.5, 0.6), 0.0, 0.0, 0, 0, Vector2(1, 0), 0.0, 0.0, 0, 0]
+point_count = 3
+
+[sub_resource type="Gradient" id="spark_grad"]
+colors = PackedColorArray(1, 0.98, 0.6, 1, 1, 1, 1, 0)
+
+[node name="bull_kick" type="CPUParticles3D"]
+emitting = false
+amount = 40
+lifetime = 1.2
+one_shot = true
+local_coords = false
+explosiveness = 0.8
+randomness = 0.5
+mesh = SubResource("dust_mesh")
+direction = Vector3(0, 0.7, -1)
+spread = 65.0
+gravity = Vector3(0, -6, 0)
+initial_velocity_min = 3.0
+initial_velocity_max = 9.0
+scale_amount_min = 0.5
+scale_amount_max = 2.2
+scale_amount_curve = SubResource("dust_curve")
+color_ramp = SubResource("dust_grad")
+script = ExtResource("oneshot")
+
+[node name="stones" type="CPUParticles3D" parent="."]
+emitting = false
+amount = 14
+lifetime = 0.65
+one_shot = true
+local_coords = false
+explosiveness = 0.92
+randomness = 0.6
+mesh = SubResource("stone_mesh")
+direction = Vector3(0, 0.35, -1)
+spread = 45.0
+gravity = Vector3(0, -9.8, 0)
+initial_velocity_min = 5.0
+initial_velocity_max = 14.0
+scale_amount_min = 0.5
+scale_amount_max = 1.8
+scale_amount_curve = SubResource("stone_curve")
+color_ramp = SubResource("stone_grad")
+
+[node name="sparks" type="CPUParticles3D" parent="."]
+emitting = false
+amount = 22
+lifetime = 0.28
+one_shot = true
+local_coords = false
+explosiveness = 0.95
+randomness = 0.4
+mesh = SubResource("spark_mesh")
+direction = Vector3(0, 0.5, -1)
+spread = 30.0
+gravity = Vector3(0, -14, 0)
+initial_velocity_min = 8.0
+initial_velocity_max = 18.0
+scale_amount_min = 0.6
+scale_amount_max = 2.0
+scale_amount_curve = SubResource("spark_curve")
+color_ramp = SubResource("spark_grad")