Restructure ESP32 firmware into modules, fix bugs, add tests

Structure (src/esp32/):
- logic/: DisplayController, schedule, power timer, LED pattern; pure C++
- display/: FlipdotFrames and bitmaps
- nano_link/: NanoLink (relay, serial, READY, bootloader) and flash bridge
- net/: WiFi/OTA/panic window, Home Assistant
- config.h with all settings; libs.h (globals in a header) removed

Bug fixes:
- Home Assistant switches now report their state back and start as ON
- loop() no longer blocks: WiFi.waitForConnectResult() (up to 60 s
  without WiFi) and getLocalTime() (5 s before NTP sync) were called
  every iteration
- Power switch off now powers off after 1 s even when a 30 s countdown
  is already running
- millis() overflow safe power-off timer; no frame catch-up bursts
- No garbage time string before NTP sync; 'Connecting...' stays instead
- Frames are only sent when the screen changes instead of every second
- Reset reasons are logged once instead of every 100 ms
- Removed dead code (unused fonts, WiFiMulti, rotated bitmap, ...)

Tests: new test_esp32 environment (26 tests); native renamed to test_nano

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

CLAUDE.md

@@ -5,9 +5,9 @@ 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/`).
+- `platformio.ini` has the environments `nano`, `nano_ota`, `esp32` (OTA to 10.1.1.198),
+  `esp32_usb` (COM11), and the PC test environments `test_nano` and `test_esp32`. 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.
@@ -19,36 +19,34 @@ See README.md for the overview.
 - `nano_ota` flashes the Nano over WiFi: `tools/nano_net_upload.py` (STK500v1 over TCP)
   talks to `NanoFlashBridge` on the ESP32 (port 2323). The uploader first sends
   `FLASH <baud>\n`. The ESP power cycles the Nano (plus an optional reset pulse on
-  `nano_reset`), answers `OK\n`, then passes the bytes through at that baud rate.
+  `NANO_RESET_PIN`), answers `OK\n`, then passes the bytes through at that baud rate.
   `--baud=auto` tries 57600 (old ATmegaBOOT, which also starts after a power cycle), then
-  115200 (Optiboot, which only starts after a reset-pin reset, so it needs `nano_reset`).
+  115200 (Optiboot, which only starts after a reset-pin reset, so it needs `NANO_RESET_PIN`).
   avrdude 6.3's `net:` port is broken on Windows (`ser_drain()` fails), which is why the
   uploader is a Python script.
   The uploader only uses the Python standard library. Test it against a simulated
   bootloader before changing it.
-- `src/esp32/libs.h` defines globals in the header, so it can only be included from
-  `main.cpp`. New ESP32 modules must be self-contained (see `nano_flash_bridge.*`).
-- After any change, build both targets (`pio run`). After Nano changes, also run
-  `pio test -e native`.
+- After any change, build both targets (`pio run`) and run the tests
+  (`pio test -e test_nano -e test_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 ESP32 talks to the Nano on `Serial2` (`nanoSerial` in `libs.h`, GPIO 17 TX / GPIO 16
-  RX). `Serial` (UART0/USB) is free for debug output. Never write debug text to
-  `nanoSerial`: the Nano runs lines that start with s/S/F/D/P/B/R/T as commands.
+- The ESP32 talks to the Nano on `Serial2` (owned by `NanoLink`, GPIO 17 TX / GPIO 16 RX).
+  `Serial` (UART0/USB) is free for debug output. Never write debug text to `Serial2`: the
+  Nano runs lines that start with s/S/F/D/P/B/R/T as commands.
 - Open hardware TODOs are listed in the README `TODO` section. Update it when one is done.
 - `src/esp32/secrets.h` holds the real credentials and is gitignored. Never commit it.
   Add new secrets to `secrets.example.h` as placeholders.
 - The relay (ESP32 GPIO 13, `LOW` = on) powers both the panels and the Nano. Each time it
-  switches on, the Nano cold-boots and then sends `READY`. The ESP's `relayOn()` waits for
+  switches on, the Nano cold-boots and then sends `READY`. `NanoLink::on()` waits for
   that message, at most `NANO_BOOT_TIMEOUT`. The timeout also covers the case where the
   Nano's TX isn't wired to the ESP's RX.
-- `relayOff()` ends `nanoSerial` and sets its TX pin to input. Otherwise the idle-high TX line
-  powers the Nano through its RX pin. Never send display commands while `relay_on` is
-  false.
+- `NanoLink::off()` ends `Serial2` and sets its TX pin to input. Otherwise the idle-high TX
+  line powers the Nano through its RX pin. Never send display commands while
+  `NanoLink::isOn()` is false (`DisplayController` takes care of this).
 - The Nano keeps no state over a power cycle except the flip time in EEPROM. The driver
   forces a full redraw on the first `display()` after boot (`_force_next`).
 - Hardware is not available here. Builds can be checked, but runtime behaviour can't.
@@ -62,19 +60,36 @@ See README.md for the overview.
   while it arrives, so it can be longer than the buffer. Keep `loop()` free of busy-waits.
 - A full redraw (`display(true)`) blocks for about 0.5 s, which overflows the 64-byte
   serial receive buffer. Only run it when no data is expected: right after a `D`, or after
-  2 s with no serial data. The ESP sends one frame per second, ending with `D`.
+  2 s with no serial data. The ESP only sends a frame (ending with `D`) when the screen
+  changes, mostly once a minute.
 - Flip time is stored in EEPROM as a magic word at address 0 and a `uint16_t` at
   address 2. Values outside 50–2000 µs are rejected.
 - The Nano only replies `READY` (at boot) and `ERR: ...`. There is no per-frame
   acknowledgement. Whether the Nano's TX is wired to the ESP's RX is unconfirmed, so every
   wait for a reply needs a timeout.
-- Unit tests: `pio test -e native` runs `test/test_nano/` on the PC with the fakes in
+- Unit tests: `pio test -e test_nano` runs `test/test_nano/` on the PC with the fakes in
   `test/mocks/` (Arduino, EEPROM, and a `BROSE9323` that records calls). It needs gcc on
   PATH (Strawberry Perl provides it at `C:\Strawberry\c\bin`). Add a test for every
   protocol change. The mocks only implement what the Nano code uses, so extend them when
   new Arduino or driver functions are called.
 - The board environments have `test_ignore = *`, so test firmware is never uploaded. Plain
-  `pio test` runs nothing; always pass `-e native`.
+  `pio test` runs nothing; always pass `-e test_nano -e test_esp32`.
+
+## ESP32 structure (`src/esp32/`)
+
+- Folders: `logic/` (pure C++, no Arduino includes, tested in `test/test_esp32`),
+  `display/` (frames and bitmaps), `nano_link/` (relay, serial, flash bridge), `net/`
+  (WiFi/OTA, Home Assistant). Includes are relative to `src/esp32` (`-I src/esp32`), e.g.
+  `#include "logic/schedule.h"`.
+- `DisplayController` holds the display logic and only uses the `DisplayPower` and
+  `Frames` interfaces. Put new behaviour there, behind those interfaces, so it stays
+  testable. `logic/` must not include Arduino headers, or `test_esp32` stops building.
+- All settings are in `config.h`. Keep it free of Arduino types for the same reason.
+- Frames are only sent when the screen changes. The schedule uses 08:00 as the start of
+  the day on purpose (that is what the old code did, and the README documents it).
+- Home Assistant switches are not optimistic: callbacks must call `sender->setState()`.
+  The unique ids `FlipDotDisplayClock` / `FlipDotDisplayPower` and `HA_DEVICE_ID` must stay
+  the same, or Home Assistant creates new entities.
 
 ## Git