Refactor and harden firmware measurement logic

[Rogger]

Summary

• extract measurement reporting policy, battery conversion, and ZCL report helpers out of main.c
• split sensor and battery update paths to reduce mixed hardware/reporting responsibilities
• harden battery ADC conversion overflow, battery ZCL attribute saturation, and Zigbee startup failure handling with regression tests

Verification

• cmake -S tests/unit -B /tmp/schneggi-unit-tests && cmake --build /tmp/schneggi-unit-tests && ctest --test-dir /tmp/schneggi-unit-tests --output-on-failure
• make build CONF_FILE=prj_debug_no_scd4x.conf OVERLAY='boards/adafruit_feather_nrf52840.overlay;boards/no_scd4x.overlay' BUILD_DIR=/tmp/schneggi-build-debug-no-scd4x
• make build CONF_FILE=prj_debug.conf OVERLAY=boards/adafruit_feather_nrf52840.overlay BUILD_DIR=/tmp/schneggi-build-debug-co2