feat(arena): add elevated-goal arena variants

Each of the three arenas gains an ELEVATED sibling scene that inherits the
base arena and overrides the boundary's goal_mode, the two goal transforms
and the ship spawns, so the goal sits at mid-wall height instead of flush
with the deck. Registered in ArenaRegistry alongside the floor-level arenas,
plus a training_elevated scene for self-play on them.

TrainingMode reads the arena's goal_mode once in _start() and widens the
ball-placement height range to match the goal's real position; on FLOOR
arenas the bound is a no-op, so floor-level training is unchanged. It is read
in _start() rather than _ready() because TrainingMode has no _ready()
override and GameMode._ready() is what discovers the arena first.

Policies trained against floor-level goals are not expected to score on an
elevated one, so the two are kept as separate arenas rather than a variant of
the same entry.

Game/scenes/arena_01_elevated.tscn

@@ -0,0 +1,20 @@
+[gd_scene load_steps=2 format=3]
+
+[ext_resource type="PackedScene" path="res://scenes/arena_01.tscn" id="1_base"]
+
+[node name="Arena" instance=ExtResource("1_base")]
+
+[node name="Boundary" parent="." index="0"]
+goal_mode = 1
+
+[node name="GoalTeam0" parent="." index="5"]
+transform = Transform3D(1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 6.0, 18)
+
+[node name="GoalTeam1" parent="." index="6"]
+transform = Transform3D(-1, 0, 0, 0, 1, 0, 0, 0, -1, 0, 6.0, -18)
+
+[node name="Spawn1" parent="SpawnsTeam0" index="0"]
+transform = Transform3D(1, 0, 0, 0, 1, 0, 0, 0, 1, 4.0, 2.8, 13.5)
+
+[node name="Spawn1" parent="SpawnsTeam1" index="0"]
+transform = Transform3D(-1, 0, 0, 0, 1, 0, 0, 0, -1, -4.0, 2.8, -13.5)

Game/scenes/arena_02_elevated.tscn

@@ -0,0 +1,20 @@
+[gd_scene load_steps=2 format=3]
+
+[ext_resource type="PackedScene" path="res://scenes/arena_02.tscn" id="1_base"]
+
+[node name="Arena" instance=ExtResource("1_base")]
+
+[node name="Boundary" parent="." index="0"]
+goal_mode = 1
+
+[node name="GoalTeam0" parent="." index="5"]
+transform = Transform3D(1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 6.0, 18)
+
+[node name="GoalTeam1" parent="." index="6"]
+transform = Transform3D(-1, 0, 0, 0, 1, 0, 0, 0, -1, 0, 6.0, -18)
+
+[node name="Spawn1" parent="SpawnsTeam0" index="0"]
+transform = Transform3D(1, 0, 0, 0, 1, 0, 0, 0, 1, 4.0, 2.8, 13.5)
+
+[node name="Spawn1" parent="SpawnsTeam1" index="0"]
+transform = Transform3D(-1, 0, 0, 0, 1, 0, 0, 0, -1, -4.0, 2.8, -13.5)

Game/scenes/arena_03_elevated.tscn

@@ -0,0 +1,20 @@
+[gd_scene load_steps=2 format=3]
+
+[ext_resource type="PackedScene" path="res://scenes/arena_03.tscn" id="1_base"]
+
+[node name="Arena" instance=ExtResource("1_base")]
+
+[node name="Boundary" parent="." index="0"]
+goal_mode = 1
+
+[node name="GoalTeam0" parent="." index="5"]
+transform = Transform3D(1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 6.0, 18)
+
+[node name="GoalTeam1" parent="." index="6"]
+transform = Transform3D(-1, 0, 0, 0, 1, 0, 0, 0, -1, 0, 6.0, -18)
+
+[node name="Spawn1" parent="SpawnsTeam0" index="0"]
+transform = Transform3D(1, 0, 0, 0, 1, 0, 0, 0, 1, 4.0, 2.8, 13.5)
+
+[node name="Spawn1" parent="SpawnsTeam1" index="0"]
+transform = Transform3D(-1, 0, 0, 0, 1, 0, 0, 0, -1, -4.0, 2.8, -13.5)

Game/scenes/training_elevated.tscn

@@ -0,0 +1,15 @@
+[gd_scene load_steps=4 format=3]
+
+[ext_resource type="Script" path="res://scripts/training_mode.gd" id="1_tr"]
+[ext_resource type="PackedScene" path="res://scenes/arena_01_elevated.tscn" id="2_tr"]
+[ext_resource type="Script" path="res://addons/godot_rl_agents/sync.gd" id="3_tr"]
+
+[node name="Training" type="Node3D"]
+script = ExtResource("1_tr")
+
+[node name="Arena" parent="." instance=ExtResource("2_tr")]
+
+[node name="Sync" type="Node" parent="."]
+script = ExtResource("3_tr")
+action_repeat = 8
+speed_up = 8.0