feat(training): support N-vs-M matches with persistent per-ship spawn IDs

Extends ShipObservations beyond the old self+1-opponent layout to padded
teammate/opponent arrays (MAX_TEAMMATES=4, MAX_OPPONENTS=5, SIZE=83),
zero-filling slots past the real roster size the same way the old single-
opponent slot was zero-filled when absent.

Slot stability across ticks requires a persistent identity: Ship gains
spawn_index (set once by GameMode.spawn_ship, never reassigned — there's no
despawn path anywhere in this codebase, so a roster is fixed for the whole
episode/match). ai_ship_controller.gd's opponent discovery is rewritten from
"first non-self ship" to classify every other ship by team and sort by
spawn_index; training_mode.gd/ship_ai_controller.gd carry the equivalent
sorted lists through the training path so both agree on slot assignment for
the same roster.

training_mode.gd and match_mode.gd both gain a team_size export (default 1,
so every existing curriculum script and match keeps today's 1v1 behaviour
unchanged). This is plumbing only: no 2v2+ curriculum or reward design, and
no match-mode UI to pick team size, has been done yet. The two checkpoints
in Game/bots/promoted/ are fitted to the old 35-float layout and are not
migrated — expected to go stale until the next training run.

Game/scripts/game_mode.gd

@@ -90,6 +90,7 @@ func spawn_ship(team: int, spawn_index: int = 0, controller: ShipController = nu
 	var spawn_transform := spawns[spawn_index] if spawn_index < spawns.size() else Transform3D.IDENTITY
 	ship.global_transform = spawn_transform
 	ship.team = team
+	ship.spawn_index = spawn_index
 	if controller:
 		ship.set_controller(controller)
 	ships.append(ship)