fix(physics): make upright a real state, and actually start ships on the floor

Six rounds of reward shaping (~700M steps) failed to produce upright ground
driving. A critical review of the simulation rather than the reward found
why:

1. The hull was a 1x1x4 box with inertia (1,1,1) and no restoring torque
   anywhere, so belly-down and rolled-90 were geometrically identical
   resting states. "Upright" was not a physically distinguished state at
   all - the reward was paying for a property the simulation did not have.
2. ~65% of episodes spawned ships via _random_position, which samples Y
   uniformly over the full 18m volume (mean ~8.7m). The measured
   airborne_fraction ~0.44 was largely that spawn distribution, and every
   ground-handling term fades out above 3m, so the shaping being tuned
   barely ever applied.
3. air_drill_chance 0.20 spawned deliberately unreachable-without-climbing
   states in the stage meant to teach ground driving, and its own
   air_touch_fraction (0.0002) shows the drills were never solved.

Fixes land in the physics and the task distribution, not the reward:
- ship.tscn: hull 1x1x4 -> 1.6x0.6x4 so it has one stable resting face;
  inertia (1,1,1) -> (7,1,7), physically correct for the hull, making
  tumbling reluctant while keeping yaw snappy.
- ship.gd: new altitude-faded righting torque (spring-damper toward
  belly-down, faded out by 3m so aerials keep full attitude freedom).
  This is the grav-plating analogue of Rocket League's auto-righting and
  helps human pilots land cleanly too.
- training_mode.gd: new ground_start_chance branch spawning ships level and
  resting on the floor with a floor-level ball - the state the handling
  stage's rewards are actually written for.
- generation5.py: ground-start-chance 0.50, air-drill-chance 0.20 -> 0.0.

Reward terms are left exactly as they were; they should finally pull in a
direction the ship can go.

Game/objects/ship.tscn

@@ -9,14 +9,14 @@ friction = 0.1
 bounce = 0.15
 
 [sub_resource type="BoxShape3D" id="BoxShape3D_dsjou"]
-size = Vector3(1, 1, 4)
+size = Vector3(1.6, 0.6, 4)
 
 [node name="Ship" type="RigidBody3D"]
 collision_layer = 1
 collision_mask = 7
 mass = 5.0
 physics_material_override = SubResource("PhysicsMaterial_ship")
-inertia = Vector3(1, 1, 1)
+inertia = Vector3(7, 1, 7)
 script = ExtResource("1_efag7")
 
 [node name="Nose" type="MeshInstance3D" parent="."]