Restructure into a single PlatformIO project with nano and esp32 targets

- Shared BROSE9323 driver in lib/, ESPSERIAL selected via build_flags
- Nano firmware in src/nano, ESP32 firmware in src/esp32
- Move WiFi/MQTT credentials to gitignored secrets.h (template provided)
- Pin home-assistant-integration to 2.1.0 from the registry
- Add esp32_usb environment for first flash / recovery
- Add READMEs and CLAUDE.md

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CLAUDE.md

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+# Flipdot display: notes for Claude
+
+A PlatformIO project with two firmware targets for an 84×19 Brose flipdot clock.
+See README.md for the overview.
+
+## Layout and build
+
+- `platformio.ini` has the environments `nano`, `esp32` (OTA to 10.1.1.198) and
+  `esp32_usb` (COM11). The sources are split with `build_src_filter` (`src/nano/`,
+  `src/esp32/`).
+- `lib/BROSE9323/` is the shared driver. Its behaviour depends on build flags set in
+  `platformio.ini`, not in the header:
+  - `ESPSERIAL` (ESP32): draw calls become serial commands.
+  - `FLIPDOT_PLCC_ADAPTER` (Nano, optional): alternative pinout.
+- PlatformIO CLI is not on PATH. Run `~/.platformio/penv/Scripts/pio.exe run` (Git Bash)
+  or `$env:USERPROFILE\.platformio\penv\Scripts\pio.exe run` (PowerShell).
+- If an ESP32 build fails with `No module named 'intelhex'`, run
+  `~/.platformio/penv/Scripts/python.exe -m pip install intelhex`.
+- After any change, build both targets: `pio run -e nano -e esp32`.
+
+## Constraints
+
+- **Nano RAM is tight** (ATmega328P, 2 KB). The frame buffers are allocated with
+  `calloc`. Avoid `String`, large stack buffers and new globals in Nano code. Put constant
+  data in `PROGMEM`.
+- The Nano and the ESP32 share one UART. On the ESP32, `Serial` is the display link, so
+  **don't print debug output on the ESP32's `Serial`**: the Nano would read it as
+  commands. The Nano ignores unknown command bytes, but text that starts with
+  s/S/F/D/P/B/R/T will be run as a command.
+- `src/esp32/secrets.h` holds the real credentials and is gitignored. Never commit it.
+  Add new secrets to `secrets.example.h` as placeholders.
+- Hardware is not available here. Builds can be checked, but runtime behaviour can't.
+
+## Known quirks in the serial protocol (Nano `src/nano/main.cpp`)
+
+- `T` waits for more than 6 bytes before parsing, but `T225\n` has only 4 after the `T`.
+  It completes only when the next command arrives.
+- The `T` value is stored with `EEPROM.update` in one byte, so values above 255 µs are
+  truncated after a reboot.
+- `P` (print text) busy-waits for a `\n` and has no timeout.
+
+## Git
+
+- Remote: https://mangavalk.com/mangavalk/Flipdot-display.git, branch `main`.