feat: add main-menu difficulty picker for Match mode

Replace the raw checkpoint dropdown with curated Easy/Medium/Hard presets
that drive GameSettings' bot model/reaction_ticks/action_noise overrides.
Move raw-checkpoint testing and Spectate mode into a dev-only section
hidden via OS.is_debug_build() so they disappear from release exports.

TODO.md

@@ -11,7 +11,7 @@ The training pipeline is built — see `TRAINING.md` (self-play PPO via the vend
 - [x] Staged curriculum (score → defend → avoid draws → full mechanics) via `train.py`'s `--opponent-mode`/`--draw-penalty`/`--attack-goal-bias`/`--vertical-ramp`/`--pitch-roll-ramp` flags — see TRAINING.md's "Curriculum training" section. `--opponent-mode=frozen` is a single-fixed-model slice of the league idea below, not the full sampled pool.
 - [ ] Frozen-opponent league: train the live policy against a *pool* of past exported checkpoints, sampled per-episode (today's `--opponent-mode=frozen` only supports one fixed model per run) to prevent self-play strategy collapse on long runs.
 - [ ] Richer state setter / curriculum: aerial states, wall plays, rebound scenarios as skill grows (beyond the score/defend/draw staging already in place).
-- [ ] Main-menu difficulty picker (Match already takes `bot_model_path`/`bot_reaction_ticks`/`bot_action_noise` exports).
+- [x] Main-menu difficulty picker: Easy/Medium/Hard presets in `main_menu.gd` set `bot_model_path`/`bot_reaction_ticks`/`bot_action_noise` via `GameSettings`. Raw-checkpoint testing and Spectate mode moved into a `DevSection` hidden outside debug builds. All three tiers currently reuse `promoted/easy.json` with different handicaps until `medium`/`hard` checkpoints are promoted.
 
 ## Multiplayer (long term)
 
@@ -22,5 +22,5 @@ The training pipeline is built — see `TRAINING.md` (self-play PPO via the vend
 
 ## General
 
-- [ ] No autoloads yet by design — add a singleton only when cross-scene state is actually needed (e.g. passing match settings/results between menu, match, and results screens).
+- [x] `GameSettings` autoload (`scripts/game_settings.gd`) added for exactly the cross-scene case this item anticipated — passing menu selections (bot/difficulty, spectate matchup) into match/spectate modes. Still the only autoload; keep it that way unless another cross-scene need shows up.
 - [ ] More arenas: `arena_01.tscn` is the template — an arena is a setting (sky, lighting, decoration) + two team-tagged goals on the standard goal lines + spawn markers + an instance of the shared `objects/arena_boundary.tscn`, with no rules or state. All arenas share the standard play volume defined by `ArenaBoundary`'s constants.