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/helpers/CommonCLI.cpp

@@ -4,6 +4,7 @@
  */
 
 #include "CommonCLI.h"
+#include "battery_curve.h"
 #include <helpers/TxtDataHelpers.h>
 #include <helpers/AdvertDataHelpers.h>
 #include <adapters/board/ZephyrBoard.h>
@@ -507,7 +508,14 @@ void CommonCLI::handleCommand(uint32_t sender_timestamp, const char* command, ch
             if (adc_mult == 0.0f) {
                 strcpy(reply, "Error: unsupported by this board");
             } else {
-                snprintf(reply, CLI_REPLY_SIZE, "> %.3f", (double)adc_mult);
+                uint16_t mv = _board->getBattMilliVolts();
+                uint16_t target_mv = battery_curve_default.ocv_mv[0];
+                if (mv > 0) {
+                    snprintf(reply, CLI_REPLY_SIZE, "> %.3f  (%u mV, target >= %u mV for 100%%)",
+                             (double)adc_mult, mv, target_mv);
+                } else {
+                    snprintf(reply, CLI_REPLY_SIZE, "> %.3f  (no ADC reading)", (double)adc_mult);
+                }
             }
         } else if (memcmp(config, "rxduty", 6) == 0) {
             snprintf(reply, CLI_REPLY_SIZE, "> %d", (int)_prefs->rx_duty_cycle);