feat(rc): CRSF/ELRS RC integration — telemetry uplink + channel fix (Issue #103)
## Summary
- config.h: CH1[0]=steer, CH2[1]=throttle (was CH4/CH3); CRSF_FAILSAFE_MS→500ms
- include/battery.h + src/battery.c: ADC3 Vbat reading on PC1 (11:1 divider)
battery_read_mv(), battery_estimate_pct() for 3S/4S auto-detection
- include/crsf.h + src/crsf.c: CRSF telemetry TX uplink
crsf_send_battery() — type 0x08, voltage/current/SoC to ELRS TX module
crsf_send_flight_mode() — type 0x21, "ARMED\0"/"DISARM\0" for handset OSD
- src/main.c: battery_init() after crsf_init(); 1Hz telemetry tick calls
crsf_send_battery(vbat_mv, 0, soc_pct) + crsf_send_flight_mode(armed)
- test/test_crsf_frames.py: 28 pytest tests — CRC8-DVB-S2, battery frame
layout/encoding, flight-mode frame, battery_estimate_pct SoC math
Existing (already complete from crsf-elrs branch):
CRSF frame decoder UART4 420000 baud DMA circular + IDLE interrupt
Mode manager: RC↔autonomous blend, CH6 3-pos switch, 500ms smooth transition
Failsafe in main.c: disarm if crsf_state.last_rx_ms stale > CRSF_FAILSAFE_MS
CH5 arm switch with ARMING_HOLD_MS interlock + edge detection
RC override: mode_manager blends steer/speed per mode (CH6)
Closes #103
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
sl-firmware
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