Add Pocket Emu handheld emulator app

.gitignore

@@ -3,3 +3,8 @@ __pycache__/
 *.pyc
 badge/.badge_state/**
 badge25/.badge_state/**
+
+# Pocket Emu app: never commit game ROMs, saves, or native build output
+badge/apps/pocket_emu/roms/*
+!badge/apps/pocket_emu/roms/.gitkeep
+tools/pocket_emu/build/

badge/apps/pocket_emu/README.md

@@ -0,0 +1,54 @@
+# Pocket Emu
+
+Play classic 8-bit handheld games from cartridges you own on the badge.
+
+![The game picker on the badge](assets/screenshot.png)
+
+Pocket Emu runs [Peanut-GB](https://github.com/deltabeard/Peanut-GB), an
+emulator written in C, as a compiled MicroPython native module (`gbcore.mpy`).
+Games run at full speed, scaled 1.5x to fill the screen, with optional colour
+palettes. The badge has no speaker, so there is no sound.
+
+## Adding games
+
+No games are included. Use ROM files (`.gb`) that you have dumped from
+cartridges you own.
+
+1. Put the badge in USB Disk Mode (open **Mass Storage**, or double-tap RESET).
+2. Copy your `.gb` files into `apps/pocket_emu/roms/` on the `BADGER` drive.
+3. Eject the drive and open **Pocket Emu** from the menu.
+
+If there is one game, it starts straight away; with several, pick one from the
+list. A typical game is 1 MB or less, and the badge has room for several.
+
+Monochrome-era games and dual-mode games that also run on monochrome hardware
+work. Colour-only games do not.
+
+## Controls
+
+| Badge | Handheld |
+| --- | --- |
+| Direction buttons or touch pad | D-pad |
+| SELECT | A |
+| BACK | B |
+| MENU (tap) | START |
+| MENU (hold) | SELECT |
+| HOME | Pause menu |
+
+The pause menu changes the colours (automatic per-game colours, classic green,
+grey), the pixel style (sharp, smooth, original size) and the speed (normal or
+2x fast forward). Choose **Save and quit** to return to the badge menu.
+
+## Saves
+
+Battery saves are written to `/state/pocket_emu/<game>.sav` on the badge about
+a second after the game writes them; **SAVED** flashes in the corner. To bring
+a save from another emulator, copy it next to the game as
+`roms/<same name as the game>.sav`; the app loads it the first time the game
+starts.
+
+## Rebuilding the emulator core
+
+`gbcore.mpy` is built from [`tools/pocket_emu/`](../../../tools/pocket_emu/).
+See that README to rebuild it after changing the C code or the firmware's
+MicroPython version.