libretro / RetroArch core

[mxmgorin]

Ship a libretro core so oxGBC runs inside RetroArch.

Scope

• New cdylib crate libretro/ (excluded from the workspace, like web), using rust-libretro (retro_core! + Core trait)
• Wire retro_get_system_av_info (160×144, ~59.73 fps, 10:9, APU rate), retro_run (frame -> video_refresh + audio_sample_batch, poll input -> joypad), serialize/unserialize -> create/load_save_state, pixel format, core options (palettes / frame blending), RetroPad input descriptors

Notes

• Ongoing cost is the multi-platform build.