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_roll_sparks.tscn

@@ -0,0 +1,42 @@
+[gd_scene format=3]
+
+; Bull roll sparks — the dirt/ember spray flung outward from the spinning roll ("Powerball").
+; Continuous: the player instances it once under the bull and toggles `emitting` during a roll.
+; World-space so the spray peels off into a trail. Tune the look here; see README.md.
+
+[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="mesh"]
+material = SubResource("mat")
+radius = 0.05
+height = 0.1
+radial_segments = 4
+rings = 2
+
+[sub_resource type="Curve" id="curve"]
+_data = [Vector2(0, 0.3), 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="grad"]
+colors = PackedColorArray(1, 0.78, 0.32, 1, 0.7, 0.42, 0.14, 0)
+
+[node name="bull_roll_sparks" type="CPUParticles3D"]
+emitting = false
+amount = 46
+lifetime = 0.45
+local_coords = false
+randomness = 0.7
+mesh = SubResource("mesh")
+direction = Vector3(0, 1, 0)
+spread = 180.0
+gravity = Vector3(0, -6, 0)
+initial_velocity_min = 3.0
+initial_velocity_max = 8.0
+scale_amount_min = 0.5
+scale_amount_max = 1.3
+scale_amount_curve = SubResource("curve")
+color_ramp = SubResource("grad")