feat(power): 1Hz chunked light-sleep; meter backoff; log throttling

- Replace delay() with light_sleep_chunked_ms() in sender idle path
  (100ms chunks preserve UART FIFO safety at 9600 baud)
- Add ENABLE_LIGHT_SLEEP_IDLE build flag (default: on, fallback: =0)
- Meter reader task: exponential backoff on consecutive poll failures
  (METER_FAIL_BACKOFF_BASE_MS..MAX_MS) to reduce idle Core-0 wakeups
- Configurable SENDER_DIAG_LOG_INTERVAL_MS (5s debug / 30s prod)
- Configurable METER_FRAME_TIMEOUT_CFG_MS, SENDER_CPU_MHZ
- New PlatformIO envs: lowpower, 868-lowpower, lowpower-debug
- Add docs/POWER_OPTIMIZATION.md with measurement plan and Go/No-Go

platformio.ini

@@ -119,3 +119,68 @@ lib_deps =
 build_flags =
   -DSERIAL_DEBUG_MODE_FLAG=0
   -DLORA_FREQUENCY_HZ=868E6
+
+; Diagnostic build: enables extended meter fault telemetry via DEBUG_METER_DIAG.
+; Use for investigating meter error rates; disable in production.
+[env:lilygo-t3-v1-6-1-diag]
+platform = https://github.com/pioarduino/platform-espressif32/releases/download/51.03.07/platform-espressif32.zip
+board = ttgo-lora32-v1
+framework = arduino
+lib_deps =
+  sandeepmistry/LoRa@^0.8.0
+  bblanchon/ArduinoJson@^6.21.5
+  adafruit/Adafruit SSD1306@^2.5.9
+  adafruit/Adafruit GFX Library@^1.11.9
+  knolleary/PubSubClient@^2.8
+build_flags =
+  -DSERIAL_DEBUG_MODE_FLAG=1
+  -DDEBUG_METER_DIAG
+
+; Power-optimised sender build: light-sleep between 1 Hz samples, serial off.
+; Use for long-duration battery-life measurements and production deployments.
+[env:lilygo-t3-v1-6-1-lowpower]
+platform = https://github.com/pioarduino/platform-espressif32/releases/download/51.03.07/platform-espressif32.zip
+board = ttgo-lora32-v1
+framework = arduino
+lib_deps =
+  sandeepmistry/LoRa@^0.8.0
+  bblanchon/ArduinoJson@^6.21.5
+  adafruit/Adafruit SSD1306@^2.5.9
+  adafruit/Adafruit GFX Library@^1.11.9
+  knolleary/PubSubClient@^2.8
+build_flags =
+  -DSERIAL_DEBUG_MODE_FLAG=0
+  -DENABLE_LIGHT_SLEEP_IDLE=1
+
+; Power-optimised + 868 MHz variant.
+[env:lilygo-t3-v1-6-1-868-lowpower]
+platform = https://github.com/pioarduino/platform-espressif32/releases/download/51.03.07/platform-espressif32.zip
+board = ttgo-lora32-v1
+framework = arduino
+lib_deps =
+  sandeepmistry/LoRa@^0.8.0
+  bblanchon/ArduinoJson@^6.21.5
+  adafruit/Adafruit SSD1306@^2.5.9
+  adafruit/Adafruit GFX Library@^1.11.9
+  knolleary/PubSubClient@^2.8
+build_flags =
+  -DSERIAL_DEBUG_MODE_FLAG=0
+  -DENABLE_LIGHT_SLEEP_IDLE=1
+  -DLORA_FREQUENCY_HZ=868E6
+
+; Power-optimised sender build with debug output enabled for validation.
+; Use during the measurement / verification phase.
+[env:lilygo-t3-v1-6-1-lowpower-debug]
+platform = https://github.com/pioarduino/platform-espressif32/releases/download/51.03.07/platform-espressif32.zip
+board = ttgo-lora32-v1
+framework = arduino
+lib_deps =
+  sandeepmistry/LoRa@^0.8.0
+  bblanchon/ArduinoJson@^6.21.5
+  adafruit/Adafruit SSD1306@^2.5.9
+  adafruit/Adafruit GFX Library@^1.11.9
+  knolleary/PubSubClient@^2.8
+build_flags =
+  -DSERIAL_DEBUG_MODE_FLAG=1
+  -DENABLE_LIGHT_SLEEP_IDLE=1
+  -DDEBUG_METER_DIAG