[raphamorim]

Adds save states to the core and two FFI helpers a host needs for save states and fast-forward.

Save states

• Gameboy::save_state() / load_state() snapshot the whole machine: CPU, WRAM/HRAM, PPU (including CGB palettes and VRAM banks), HDMA, timer, joypad, serial, sound channels, and the cartridge's banking registers, RAM and RTC.
• New src/state.rs holds a small binary format, with no new dependencies. Each component lists its fields in order through an impl_state! macro.
• Header: magic, a format version, and the cartridge's header and global checksums. A state for another game is rejected.
• A load that fails partway rolls back, so the running game is never left half-restored.
• Sound sample buffers are cleared on load rather than saved. The audio player and serial callback aren't part of the state.

FFI

• save_state(path) writes through a temp file plus rename (reusing the save-file writer, whose temp name now works for any extension). load_state(path) restores a state.
• set_audio_muted(bool) drops samples while set, and clears the queue when muting starts. Hosts use it to fast-forward without building up seconds of audio lag.

Other

• Silences the unused-variable warnings in CpalPlayer::new, which appear in release builds because the errors are only printed with debug assertions.

Tests

• Two unit tests with a synthetic ROM: a restored state replays bit-identically, and other games', truncated and garbage states are rejected without touching the running game.
• Tried in the Fliper macOS app with Pokémon Silver:
  • quick save and quick load
  • resume after quitting
  • fast-forward