feat(*): Replace the test terrain arena with an enclosed standard-size space-platform arena (shared ArenaBoundary floor/walls/ceiling scene, starfield sky, ball CCD) and derive TrainingMode placement bounds from it, dropping the out-of-bounds reward guard

CLAUDE.md

@@ -40,7 +40,7 @@ The structure was deliberately chosen so an RL-trained AI opponent and, later, m
 
 - **Scene flow**: `scenes/main_menu.tscn` (`main_menu.gd`, one handler per mode) → `scenes/free_play.tscn` (practice: no timer, R resets ball) or `scenes/match.tscn` (150s timer, per-team score, kickoff resets). Esc returns to the menu from either mode.
 - **Controller seam (do not bypass)**: `Ship` (`scripts/ship.gd`, `RigidBody3D`) never reads `Input`. Each physics tick, `_integrate_forces` pulls one `ShipAction` (`scripts/ship_action.gd`: thrust `Vector3`, rotation `Vector3`, turbo `bool`, each axis -1..1) from its `ShipController` child (`scripts/ship_controller.gd`, base returns a zero action). `PlayerShipController` reads input actions; a future `AIShipController` (RL policy) or network-replication controller implements the same `get_action()` interface. A ship with no controller is inert but simulated. The ShipAction shape *is* the future RL action space — change it deliberately.
-- **Arena vs game mode**: `scenes/arena_01.tscn` (`scripts/arena.gd`, group `"arena"`) is a stateless stadium — terrain, lighting, two `Goal` instances (team 0 and 1), `BallSpawn` and `SpawnsTeam0/1` Marker3Ds — queried via `get_ball_spawn()`/`get_ship_spawns(team)`/`get_goals()`. Game modes extend `GameMode` (`scripts/game_mode.gd`, group `"game"`): the mode's scene contains an Arena + HUD, and the mode spawns ball/ships/controllers/camera **in code** (`spawn_ship(team, index, controller)` etc.) so ship counts and controller mixes stay flexible. `free_play.gd` and `match_mode.gd` override `_start()` and `_on_goal_scored(conceding_team)`.
+- **Arena vs game mode**: `scenes/arena_01.tscn` (`scripts/arena.gd`, group `"arena"`) is a stateless stadium — a setting (space-platform floor, starfield sky, lighting), an enclosing `Boundary` (instance of `objects/arena_boundary.tscn`: floor/walls/ceiling colliders), two `Goal` instances (team 0 and 1), `BallSpawn` and `SpawnsTeam0/1` Marker3Ds — queried via `get_ball_spawn()`/`get_ship_spawns(team)`/`get_goals()`. All arenas are a standard size: they instance the shared `arena_boundary.tscn`, and `scripts/arena_boundary.gd` (`ArenaBoundary`) holds the canonical play-volume constants (inner x ±12, z ±18, height 12, goal lines z ±17) that field-size logic must derive from instead of restating numbers. Game modes extend `GameMode` (`scripts/game_mode.gd`, group `"game"`): the mode's scene contains an Arena + HUD, and the mode spawns ball/ships/controllers/camera **in code** (`spawn_ship(team, index, controller)` etc.) so ship counts and controller mixes stay flexible. `free_play.gd` and `match_mode.gd` override `_start()` and `_on_goal_scored(conceding_team)`.
 - **Goals are dumb sensors**: `scripts/goal.gd` (`Area3D`, group `"goal"`, `@export team`) only emits `goal_scored(team)` when a body in group `"ball"` enters; `GameMode` debounces it (`_handle_goal_scored`) and modes decide consequences. Never put scoring/reset logic in the goal.
 - **Ship physics**: all movement is force/torque-based (`_integrate_forces`), not kinematic — inputs become world-space forces/torques relative to ship orientation, with manual drag and speed clamps per tick. Physics formulas are commented inline; see `FLIGHT_MANUAL.md` for the player-facing flight model. Physics properties (mass, inertia, friction material) live in `objects/ship.tscn`, not in `_ready` overrides — keep the scene truthful; RL tuning depends on it.
 - **Camera** (`scenes/ship_camera_rig.tscn`, `scripts/ship_camera.gd`, group `"ship_camera"`) is spawned by the game mode and given a `target` ship — ships have no camera/HUD dependency, so headless RL runs work (`godot --headless`).