feat: replace ship and ball placeholder meshes with Blender-modeled assets

Ship gets a greebled hull, tapered nose, swept canopy, twin engine nacelles,
and tail fin (built via the vendored Blender MCP, generator committed at
tools/blender/gen_ship.py) in place of the 5 flat primitives. The ball is
fully remodeled as a smooth round sphere with a crossed emissive accent
pattern (gen_ball.py), replacing the old flat-shaded gold_ball rather than
just tweaking its material.

Node names (Nose/TailFin) are preserved for Ship._apply_team_color(), and
the RigidBody3D/CollisionShape3D physics on both ship.tscn and ball.tscn are
untouched so RL-trained bots and flight feel stay valid. Each part's mesh is
extracted to a standalone .res (tools/blender/extract_meshes.gd) rather than
referenced via glb::ArrayMesh_xxx sub-paths, which don't reliably resolve
across scene files and were silently rendering both models invisible.

Game/objects/ball.tscn

@@ -1,6 +1,6 @@
 [gd_scene load_steps=5 format=3 uid="uid://27u3tdc5yqnl"]
 
-[ext_resource type="ArrayMesh" uid="uid://ucw1lyo43yi4" path="res://assets/models/gold_ball.res" id="1_ct1s3"]
+[ext_resource type="ArrayMesh" path="res://assets/models/ball.res" id="1_ball"]
 [ext_resource type="Script" path="res://scripts/ball.gd" id="2_ball"]
 
 [sub_resource type="SphereShape3D" id="SphereShape3D_c5p07"]
@@ -24,5 +24,5 @@ metadata/_edit_group_ = true
 shape = SubResource("SphereShape3D_c5p07")
 
 [node name="MeshInstance3D" type="MeshInstance3D" parent="."]
-transform = Transform3D(0.5, 0, 0, 0, 0.5, 0, 0, 0, 0.5, 2.08165e-12, 2.08165e-12, 2.08165e-12)
-mesh = ExtResource("1_ct1s3")
+transform = Transform3D(0.5, 0, 0, 0, 0.5, 0, 0, 0, 0.5, 0, 0, 0)
+mesh = ExtResource("1_ball")