battery: centralized LiPo OCV curve + ADC multiplier corrections

Add a board-aware battery SOC system replacing the two independent
linear approximations that existed in the UI helpers.

New helpers/battery_curve.{h,c}: a 21-point (5% step) OCV lookup table
with integer linear interpolation. The default generic LiPo curve is a
weak symbol — any board can override it by dropping a battery_curve.c
into its board directory. CMakeLists.txt selects the board-specific file
when present, falling back to the generic.

Board.h gains getBattPercent() (default 0); ZephyrBoard implements it
via battery_curve_lookup().

Board-specific curves added for boards with measured cell data:
t1000_e, rak_wismesh_tag, thinknode_m6, sensecap_solar, wio_tracker_l1.

ADC multiplier corrections applied across all nRF52840 boards:
- Boards using a correctly-derived 3600×ratio formula get +0.5% to
  compensate for nRF SAADC gain error (7200→7236, 6300→6332, etc.)
- rak_wismesh_tag, rak3401_1watt, gat562_30s had multipliers copied
  from Arduino's 3.0V AREF formula; corrected to 3600×1.73×1.005=6259
- xiao_nrf52840 (10911) left unchanged — empirically calibrated value
  above the theoretical, assumed already correct for that hardware

get adc.multiplier now reports current mV reading and the board's
curve 100% target, making field calibration self-guiding.

zephcore/include/mesh/Board.h

@@ -17,6 +17,7 @@ namespace mesh {
 class MainBoard {
 public:
 	virtual uint16_t getBattMilliVolts() = 0;
+	virtual uint8_t  getBattPercent() { return 0; }
 	virtual float getMCUTemperature() { return NAN; }
 	virtual bool setAdcMultiplier(float multiplier) { (void)multiplier; return false; }
 	virtual float getAdcMultiplier() const { return 0.0f; }