Menu: Gamepad menu combos

Depends on the menu system from #129 / PR #169. Complements the rebindable-hotkeys issue.

Context

The menu currently opens with F1 or the gamepad Guide button, but many generic controllers have no Guide button, leaving gamepad-only setups unable to open the menu. raylib-libretro offers selectable button combos (LibretroMenuCombo): Select+Start, L1+R1, L2+R2, L3+R3, Down+Select, checked per-frame via LibretroMenuComboTriggered().

Proposed design

• A Menu Combo combobox in Settings (None|Select+Start|L1+R1|L2+R2|L3+R3|Down+Select).
• Check the combo in SDL_Libretro_UpdateMenu() using SDL_GetGamepadButton over the open gamepads (or nk_gamepad state), with edge detection so it fires once per press.
• Suppress the frame's combo buttons from reaching the core when the combo triggers, so the game doesn't also react.
• Persist the selection in the config (menucombo).

Tasks

• Combo enum + per-frame trigger detection
• Settings combobox + config persistence
• Input suppression when the combo fires
• test_Menu coverage with synthetic gamepad state where feasible
• README update

References

• raylib-libretro include/raylib-libretro-menu.h: LibretroMenuCombo, LibretroMenuComboTriggered()

Start with SDL_Libretro_UpdateMenu(), the raylib-libretro menu header reference, and the existing Settings/config flow. Review SDL_GetGamepadButton or nk_gamepad state and test_Menu for the relevant input behavior. Done means the listed combos trigger once, suppress their buttons for that frame, persist through config, have feasible test coverage, and are documented in README.