Add serial dev commands and a serial log helper

The firmware accepts burst, key <name>, sound on|off and short|normal
over serial, so UI behaviour can be reproduced on the device without
the keyboard. scripts/serial_log.sh records (and drives) it in a named
container that flash.sh removes before uploading.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

README.md

@@ -36,3 +36,16 @@ This runs the host-side unit tests (`test/`, `native` environment), then builds
 ```sh
 sudo usermod -aG dialout "$USER"
 ```
+
+## Development aids
+
+`scripts/serial_log.sh [seconds] [command…]` records the serial output, and can send commands to the firmware first. For example, `scripts/serial_log.sh 30 short sleep:12 burst` sets short screen timeouts, waits 12 s, then sends a burst of Toasts.
+
+| Command | Effect |
+|---|---|
+| `burst` | Publishes 5 Notifications at once |
+| `key up\|down\|left\|right\|select\|back\|home` | Injects a logical key press |
+| `sound on` / `sound off` | Toggles the Sound setting (beep + LED) |
+| `short` / `normal` | Screen timeouts 5 s / 10 s, or 30 s / 60 s |
+
+`scripts/flash.sh` stops a running serial log first, since it would hold the port.