feat: battery/charge status and analog trigger+stick calibration (PR 5/9)

[thePunderWoman]

Summary

PR 5 of the firmware rewrite (PR 1: #35, PR 2: #36, PR 3: #38, PR 4: #39). Bundles three subsystems that share the same shape of problem — ADC reading + calibration.

• BatteryMonitor (include/battery.h / src/battery.cpp): raw VSYS ADC → battery percentage via the R_VSYS1/R_VSYS2 divide-by-3 network and a linear approximation between empty/full cell voltage (flagged as a v1 simplification of a real Li-ion discharge curve — easy to refine later). Charge-state decoding from STAT1/STAT2 is pulled directly from the bq25185 datasheet's (SLUSF65A) Table 7-2 "Status Pins State Table" rather than guessed at.
• AnalogCalibration (include/calibration.h / src/calibration.cpp): raw trigger/stick ADC → calibrated 0-100 / -100..100 using stored min/max/center, with a small deadzone around stick center so a resting stick reads exactly 0.
• TriggerCalibrationFlow / StickCalibrationFlow: pure guided-calibration state machines (release-then-pull for the trigger; center-then-roll for the stick). Not wired to any UI yet — that's the menu system in PR 6/7 — but the reusable logic exists and is fully tested now.
• CalibrationStore (src/calibration_store.cpp): thin Preferences/NVS adapter persisting calibration data, joining oled.cpp/rgb_led.cpp in the native build/coverage exclusion list.

SnipsController.ino reads all four analog channels plus STAT1/STAT2 once a second and logs the calibrated/decoded results — the packet protocol to actually transmit this over XBee lands in PR 8.

Test plan

• pio test -e native — all 65 test cases pass
• gcovr --exclude 'src/SnipsController\.ino' --exclude 'src/oled\.cpp' --exclude 'src/rgb_led\.cpp' --exclude 'src/calibration_store\.cpp' --fail-under-line 90 — 100% line coverage across all logic files
• pio run -e esp32s3 — builds successfully
• Flash to real hardware once boards arrive: confirm battery %/charge state and trigger/stick readings look sane over serial; note trigger/stick will read the uncalibrated full ADC range until the guided calibration UI exists (PR 6/7)

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