Add RFD900x remote reconfiguration via uplinked RfdConfig (Phase 4)

[ivan-lyf]

What

Implements the missing phase of two-way radio communication: FALCON can now reconfigure the rocket-side RFD900x over serial from an uplinked RfdConfig protobuf command (+++ -> ATSn=v per populated field -> AT&W -> ATZ), per the team decision that FALCON applies rfd_config (superseding the stale "ground-local" comment in GroundCommand.proto).

Safety design — this must never brick comms

• Validate before touching the modem: range checks per S-register plus min_freq <= max_freq; an empty or invalid config is rejected with no bytes sent (-EINVAL).
• Every command verified: each ATSn=v must return OK, then the register is read back with ATSn? and compared — a silently-clamped value aborts the session.
• AT&W is only sent after every write verified; ATZ only after AT&W returns OK. Any earlier failure (timeout, ERROR, mismatch) aborts with ATO, so EEPROM keeps the config the link was established with.
• Telemetry TX suspended during the session (radio_tx_suspend()) so the +++ guard time sees a silent serial line toward the modem.
• Robust response parsing: command echo, blank lines, and RSSI noise are ignored; only a bare OK / all-digit value line counts.

Structure

• firmware/src/radio/rfd900x_at.c — transport-agnostic AT session engine (the part under test)
• firmware/src/radio/rfd900x.c — UART transport + telemetry suspension glue; simulated on native_sim
• firmware/src/radio/command_thread.c — dispatches GroundCommand.rfd_config to the engine
• boards/.../ubcrocket_polarity.dts — rfd-uart alias, PLACEHOLDER: USART2 PA2/PA3 @ 57600 assumed; if the real path is AT-passthrough via the GNSS board, only the transport in rfd900x.c changes

Testing

New suite firmware/tests/rfd900x runs the engine against a scripted mock modem — 12/12 passing on native_sim/native/64:
happy path (all 6 registers), partial config, zero-value field (NETID 0), echo/noise tolerance, empty config, min>max, out-of-range values, no OK after +++, ERROR reply, mid-sequence silence, readback mismatch, unacknowledged AT&W (asserts ATZ never sent).

Also verified: full firmware builds for ubcrocket_polarity (SDK 0.17.4) and native_sim/native/64; existing state-machine suite still passes; integration suite still builds for ubcrocket_polarity; clang-format clean.

Open items before flight

• Confirm the physical serial path to the modem (direct UART vs GNSS-board passthrough) and the final S-register bounds in uplink_config.h (currently placeholders).
• Bench-test against two real RFD900x modems, including the ground-station sequencing/recovery-window plan (rocket reconfigures first, ground follows).
• The final command list from the finalized protobuf may add/remove registers — extend build_writes() accordingly.

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