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_dash.tscn

@@ -0,0 +1,44 @@
+[gd_scene format=3]
+
+; Bull dash burst — the one-shot streak of speed shards left when a dash/roll launches. Fired
+; via Fx.spawn(&"bull_dash", xform); the spawn transform aims local -Z along the dash direction,
+; so the shards streak the right way. Tune the look here; see systems/fx/README.md.
+
+[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="BoxMesh" id="mesh"]
+material = SubResource("mat")
+size = Vector3(0.045, 0.045, 0.3)
+
+[sub_resource type="Curve" id="curve"]
+_data = [Vector2(0, 1), 0.0, 0.0, 0, 0, Vector2(1, 0), 0.0, 0.0, 0, 0]
+point_count = 2
+
+[sub_resource type="Gradient" id="grad"]
+colors = PackedColorArray(0.5, 0.82, 1, 1, 0.5, 0.82, 1, 0)
+
+[node name="bull_dash" type="CPUParticles3D"]
+emitting = false
+amount = 18
+lifetime = 0.22
+one_shot = true
+local_coords = false
+explosiveness = 0.92
+randomness = 0.15
+mesh = SubResource("mesh")
+direction = Vector3(0, 0, -1)
+spread = 15.0
+gravity = Vector3(0, 0, 0)
+initial_velocity_min = 18.0
+initial_velocity_max = 35.0
+scale_amount_min = 0.8
+scale_amount_max = 2.8
+scale_amount_curve = SubResource("curve")
+color_ramp = SubResource("grad")
+script = ExtResource("oneshot")