feat(arena): resize for 5v5, cut a real goal-wall hole, and bake collision geometry

Retraining from scratch removes the constraint that blocked these: the
existing checkpoints in Game/bots/promoted/ no longer need byte-identical
geometry.

- Fix the goal aperture constants (were 1.85/1.65, now match
  objects/goal.tscn's actual sensor exactly at 1.75/1.5) so a ball crossing
  the visible edge can't fail to score.
- Cut a real navigable hole in each end wall's collision
  (_build_end_wall_colliders), replacing the previous solid box — the ball
  now genuinely enters the net instead of triggering the sensor a hair
  before hitting a solid wall. Correct for both FLOOR and ELEVATED goal
  modes via _goal_surround_bounds. A separate, slightly wider
  GOAL_VISUAL_APERTURE_* pair keeps the collision hole exact while still
  giving goal.gd's bezel/rim frame clearance to be seen against the hull cut.
- Resize the play volume 1.5x (INNER_HALF_X/Z/HEIGHT 12/18/12 -> 18/27/18) to
  comfortably fit a 5v5 roster: updates every hand-authored literal in
  arena_boundary.tscn/arena_base.tscn/the elevated arena variants that
  doesn't derive from those constants, adds 5 spawn markers per team, and
  rescales ship_observations.gd's normalization scales and arena_02's
  cosmetic decoration/nebula-dust shader uniforms to match.
- Bake the ~170 runtime-generated CollisionShape3D nodes into the scene via
  a new bake_colliders()/tools/bake_arena_boundary.gd instead of rebuilding
  them on every load — a real load-time cost repeated in every parallel
  headless training env. Colliders must be direct children of the
  StaticBody3D to register at all, so bake_colliders() parents them onto
  self and tags them with a group for idempotent re-baking, rather than
  grouping them under an intermediate container node. _ready() self-heals:
  it skips regenerating only when an existing bake's goal_mode metadata
  matches the current one, so the FLOOR-mode bake in the shared scene is
  never silently reused by an ELEVATED arena variant.

Game/scenes/arena_base.tscn

@@ -53,16 +53,16 @@ shadow_enabled = false
 environment = SubResource("Environment_base")
 
 [node name="ReflectionProbe" type="ReflectionProbe" parent="."]
-transform = Transform3D(1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 6, 0)
-size = Vector3(26, 14, 40)
+transform = Transform3D(1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 9, 0)
+size = Vector3(38, 20, 58)
 box_projection = true
 update_mode = 0
 
 [node name="GoalTeam0" parent="." instance=ExtResource("6_p57ef")]
-transform = Transform3D(1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0.79, 18)
+transform = Transform3D(1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0.79, 27)
 
 [node name="GoalTeam1" parent="." instance=ExtResource("6_p57ef")]
-transform = Transform3D(-1, 0, 0, 0, 1, 0, 0, 0, -1, 0, 0.79, -18)
+transform = Transform3D(-1, 0, 0, 0, 1, 0, 0, 0, -1, 0, 0.79, -27)
 team = 1
 
 [node name="BallSpawn" type="Marker3D" parent="."]
@@ -71,9 +71,33 @@ transform = Transform3D(1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 2, 0)
 [node name="SpawnsTeam0" type="Node3D" parent="."]
 
 [node name="Spawn1" type="Marker3D" parent="SpawnsTeam0"]
-transform = Transform3D(1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 2.8, 13.5)
+transform = Transform3D(1, 0, 0, 0, 1, 0, 0, 0, 1, -9, 2.8, 20.25)
+
+[node name="Spawn2" type="Marker3D" parent="SpawnsTeam0"]
+transform = Transform3D(1, 0, 0, 0, 1, 0, 0, 0, 1, -4.5, 2.8, 20.25)
+
+[node name="Spawn3" type="Marker3D" parent="SpawnsTeam0"]
+transform = Transform3D(1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 2.8, 20.25)
+
+[node name="Spawn4" type="Marker3D" parent="SpawnsTeam0"]
+transform = Transform3D(1, 0, 0, 0, 1, 0, 0, 0, 1, 4.5, 2.8, 20.25)
+
+[node name="Spawn5" type="Marker3D" parent="SpawnsTeam0"]
+transform = Transform3D(1, 0, 0, 0, 1, 0, 0, 0, 1, 9, 2.8, 20.25)
 
 [node name="SpawnsTeam1" type="Node3D" parent="."]
 
 [node name="Spawn1" type="Marker3D" parent="SpawnsTeam1"]
-transform = Transform3D(-1, 0, 0, 0, 1, 0, 0, 0, -1, 0, 2.8, -13.5)
+transform = Transform3D(-1, 0, 0, 0, 1, 0, 0, 0, -1, -9, 2.8, -20.25)
+
+[node name="Spawn2" type="Marker3D" parent="SpawnsTeam1"]
+transform = Transform3D(-1, 0, 0, 0, 1, 0, 0, 0, -1, -4.5, 2.8, -20.25)
+
+[node name="Spawn3" type="Marker3D" parent="SpawnsTeam1"]
+transform = Transform3D(-1, 0, 0, 0, 1, 0, 0, 0, -1, 0, 2.8, -20.25)
+
+[node name="Spawn4" type="Marker3D" parent="SpawnsTeam1"]
+transform = Transform3D(-1, 0, 0, 0, 1, 0, 0, 0, -1, 4.5, 2.8, -20.25)
+
+[node name="Spawn5" type="Marker3D" parent="SpawnsTeam1"]
+transform = Transform3D(-1, 0, 0, 0, 1, 0, 0, 0, -1, 9, 2.8, -20.25)