Firmware Host Syntax Check

Snapshot of tools/fw_syntax_check/README.md at commit 707b4a0, preserved before the docs were compacted. History only: the current specs are in the repository.

Firmware host syntax check

Compiles the firmware .cpp files with the host g++ (-std=gnu++17 -fsyntax-only -Wall -Wextra) against stub headers, so that syntax and type errors show up without PlatformIO (seconds instead of a 4 min build).

python3 tools/fw_syntax_check/check.py                       # all of firmware/src, exit code 1 on errors
python3 tools/fw_syntax_check/check.py firmware/src/Block/LEDBlock/LEDBlock.cpp   # single files
python3 tools/fw_syntax_check/check.py -j 8 -v               # 8 parallel jobs, list OK files too

Needs only g++ (13 tested) and Python 3.

• stubs/ holds Arduino core (Arduino.h, WString.h, Print.h, HWCDC.h), ESP-IDF 4.4 (esp_*.h, freertos/), classic BLE (BLEDevice.h and aliases), SPIFFS.h / FS.h, Preferences.h, WiFi.h and Adafruit_NeoPixel.h.
• stubs/ArduinoJson.h is the real single-header release 7.4.2 (MIT, from the GitHub release), the version that firmware/platformio.ini resolves ^7.4.2 to. Update it together with lib_deps.
• Files that mention REACTPOD_BLE_ESPNOW_TEST are checked a second time with that flag (env BleEspNowTest).
• A missing name or signature in a stub is a stub gap: fix the stub, not the firmware. Add only what the firmware uses.

Limits: names and signatures only, no linking, no runtime behaviour. The host is 64-bit (int64_t is long, uint32_t is unsigned int), so -Wformat warnings about %lld / %lu can differ from the RISC-V build. sdkconfig macros and Arduino core version differences (espressif32@6.10.0) are not modelled; the real build stays the final check.