New Feature: Dual switchable receivers

[xznhj8129]

Dual RX: primary/secondary receivers with automatic failover

WIP, requires more elaboration on switchover states, hysteresis and dwell time, receiver configurations and telemetry

Summary

Adds a second receiver that runs alongside the primary. Both links stay active for
telemetry, but only one drives RC control at a time. When the primary link enters
failsafe, control passes to the secondary; the main failsafe only triggers when
both links are down.

• Gated by dual_rx_enabled (default OFF); auto-disabled if either link is an MSP RX
  (MSP RC can arrive on any MSP port, so a dedicated secondary link makes no sense there).
• A second identical serial protocol can bind to its own UART via the new
  FUNCTION_RX_SERIAL_SECONDARY serial function.
• OSD gains an active-link indicator and a "primary in failsafe" warning.
• Unit tests cover primary→secondary failover, secondary-active startup, and
  main-failsafe-only-when-both-down.
• Based on the last stage of Mavlink_multiport branch

Configuration / CLI

New gate and secondary receiver settings in PG_RX_CONFIG:

• dual_rx_enabled — bool, default OFF.
• receiver_type_secondary — default NONE.
• serialrx_provider_secondary — default per target.
• serialrx_inverted_secondary — default OFF.
• serialrx_halfduplex_secondary — default AUTO.
• sbus_sync_interval_secondary — default 3000 µs (range 500–10000).

Serial function masks (assigned with serial <idx> <mask> ..., additive when a port is shared):

• Primary: FUNCTION_RX_SERIAL (64)
• Secondary: FUNCTION_RX_SERIAL_SECONDARY (8192 = 1 << 13, reusing the previously
  unused serial-function bit)

Runtime / selection / failsafe

• rxLink_e abstraction with per-link runtime state (channels, link quality, stats,
  timing) maintained every cycle for both PRIMARY and SECONDARY, plus the APIs
  rxGetActiveLink, rxSetActiveLink, rxIsLinkReceivingSignal, rxIsPrimaryFailsafe.
• Selection: primary while it has a valid signal, else secondary; if neither is valid,
  primary stays selected but marked invalid. Main failsafe fires only when both links
  are down.
• Serial RX drivers take a port-function argument so they bind to the primary or
  secondary UART; CRSF writes link statistics through the per-link runtime pointer.
• LOGIC_CONDITION_RX_SELECT_OVERRIDE (operation 63) lets the programming framework
  force the active link (PRIMARY/SECONDARY) via the OVERRIDE_RX_LINK global flag.

OSD / UI

• New OSD_ACTIVE_RX_LINK element (default position (3,5), CMS-exposed) showing RX:1
  or RX:2; blinks when running on the secondary link or when no link is valid.
• New multifunction warning "PRI RX FS" shown when the primary is in failsafe while the
  secondary still has signal. The multifunction warning bitfield was widened from 8 to
  16 bits to accommodate the added warning.

MSP

• MSP2_INAV_MISC reply appends a trailing activeRX byte (rxLink_e) reporting the
  current active link; older clients that stop reading before it are unaffected.

Known limitations

• Two identical receivers work for CRSF and SBUS only (for now). Those two drivers hold
  per-link parser/channel state, so a primary and secondary receiver of the same type run
  independently. The remaining serial drivers (Spektrum, SUMD, IBUS, JETI, FPORT/FPORT2,
  SRXL2, GHST) still keep parser state in shared module storage, so running the same one
  of those on both links would corrupt both. rxInit() therefore refuses dual RX only when
  both links are SERIAL with the same, not-yet-instance-safe serialrx_provider (it falls
  back to single-RX), mirroring the existing MSP guard. Two different providers always run
  independently. Making the remaining drivers instance-safe (same pattern as CRSF/SBUS) is a
  follow-up.
• No fail-back hysteresis. Selection returns to the primary as soon as it regains a
  valid signal, with no dwell time or blending across the switch. A marginal primary link
  can cause the active link to flap frame-to-frame; a hold-off timer is a sensible future
  refinement.
• PG_RX_CONFIG version bumped (13 → 14) because the new secondary fields change the
  stored rxConfig_t layout — RX config resets to defaults on upgrade.

Docs

• docs/Settings.md and the MSP enum/message docs (docs/development/msp/) regenerated.

Configurator companion

The INAV Configurator companion lives on feature/dualrx (based on mavlink_multiport2)
and adds the secondary-receiver UI, the RX_SERIAL_SECONDARY port function, the
activeRX MSP read, and the "Select RX link" programming operator (id 63).

Tests / verification

• rx_rx_unittest (RxDualLinkTest) rewritten for the dual-link API — 3/3 pass.
• SITL builds clean with warnings-as-errors.
• Hardware sanity: SPEEDYBEEF405WING builds and fits (FLASH 72.67%, RAM 93.01%).
• Full test pending

Build/run the unit test:

cmake -S . -B build_unit -DTOOLCHAIN=
cmake --build build_unit --target rx_rx_unittest
./build_unit/src/test/unit/rx_rx_unittest