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/adapters/board/ZephyrBoard.cpp

@@ -3,6 +3,7 @@
  */
 
 #include "ZephyrBoard.h"
+#include "battery_curve.h"
 #include <zephyr/kernel.h>
 #include <zephyr/sys/reboot.h>
 #include <zephyr/drivers/sensor.h>
@@ -146,6 +147,11 @@ uint16_t ZephyrBoard::getBattMilliVolts()
 #endif
 }
 
+uint8_t ZephyrBoard::getBattPercent()
+{
+	return battery_curve_lookup(&battery_curve_default, getBattMilliVolts());
+}
+
 bool ZephyrBoard::setAdcMultiplier(float multiplier)
 {
 #if DT_NODE_EXISTS(DT_PATH(zephyr_user)) && \