fix(led): stop WS2812 refresh when frame is unchanged so 'off' stays dark

[mitchross]

Fixes #65

Problem

With brightness set to off, the LEDs blink every ~10-17 seconds (visible in a dark room). Serial console showed the real root cause — a reboot loop:

I (13633) zigbee: Waiting for coordinator (ESP_FAIL), attempt 4/5, retrying
E (16993) zigbee: Zigbee init failed 5 times – rebooting to reset radio
ESP-ROM:esp32h2-eco5-20240613
rst:0xc (SW_CPU),boot:0x1c (SPI_FAST_FLASH_BOOT)

A commissioned device whose coordinator is unreachable (powered off, out of range, network gone) fails ESP_ZB_BDB_MODE_INITIALIZATION 5 times and then calls esp_restart() — which can't help, so it boot-loops forever. Each boot flashes the LED before button_init() loads the saved brightness from NVS, so a device set to off blinks all night. Likely also related to #54 and #42.

Fixes

zigbee.c — stop the reboot loop

• Reboot at most once per power cycle to clear a genuinely wedged radio (skip when the last reset was already ESP_RST_SW — it didn't help).
• After that, keep retrying initialization with exponential backoff (1s → 2s → 4s … capped at REJOIN_BACKOFF_MAX_MS = 5 min) instead of restarting.

led.c — no boot flash when brightness is off

• Start led_intensity at 0 so the LEDs stay dark until button_init() loads the saved brightness.

led.c — idle data line when off

• Only write to the WS2812 strip when the frame changes (smooth animations still update at 50 Hz), repeat writes for 500 ms after a change so a corrupted frame self-heals, refresh at 1 Hz while lit, and keep the line fully idle once settled at off.

Hardware verification (ESP32-H2, built with IDF v5.5.2)

Flashed and watched serial for 3 minutes with the coordinator unreachable:

E (16793) zigbee: Zigbee init failed 5 times – rebooting once to reset radio
rst:0xc (SW_CPU)   <- single reboot, then no further restarts
I (20219) zigbee: Waiting for coordinator (ESP_FAIL), attempt 6, retrying in 2000 ms
I (24579) zigbee: Waiting for coordinator (ESP_FAIL), attempt 7, retrying in 4000 ms
I (41299) zigbee: Waiting for coordinator (ESP_FAIL), attempt 9, retrying in 16000 ms
I (160379) zigbee: Waiting for coordinator (ESP_FAIL), attempt 12, retrying in 128000 ms

Sensors, serial JSON, and BLE keep running throughout. Overnight dark-room test of the off-LED behavior in progress; will report back.

Flashing for testing

Built with espressif/idf:v5.5.2, idf.py set-target esp32h2 build, then merged to a single image:

esptool.py --chip esp32h2 merge_bin -o AirCube_firmware_led_fix.bin @flash_args
esptool.py --chip esp32h2 --port /dev/cu.usbmodemXXXX --baud 460800 write-flash 0x0 AirCube_firmware_led_fix.bin

(Equivalent to the ESP Launchpad DIY flow at address 0x0 from FIRMWARE_UPDATE.md.)

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