[pfeerick]

Summary

On the Flysky radios (PL18, PL18EV, PL18U, NB4P, NV14, EL18, PA01, ST16) the trainer jack has input on the tip and output on the ring. That suits the crossover lead (tip↔ring) these radios ship with. A mono lead grounds the ring, so as slave they send nothing. That's the problem in #6970. A tip-only radio has the same issue over a straight stereo lead. Pulling the plug part-way out "fixing" it is a symptom of this: the plug tip lands on the ring contact.

#6970 moves the output to the tip on every board. That fixes mono leads but breaks pairing over the supplied crossover lead. This PR makes the wiring a setting instead.

Changes

• New hw_defs flag: trainer_jack_crossover. The radio setting is shown only on boards that set this flag.
• New radio setting: Radio settings → Hardware → Trainer lead, stored in a spare COLORLCD bit (trainerTipOnly).
  • Crossover (default): input on the tip, output on the ring, as before. This works with the supplied lead.
  • Standard: both input and output on the tip, for mono or straight stereo leads and radios that only use the tip.
• Live switching: changing the setting restarts the trainer with no reboot needed.
• Companion: Companion supports the option.
• ST16 now behaves the same as the other Flysky radios. Its trainer input and output were the reverse of its siblings: output on the tip, input on the ring. So it couldn't pair with another Flysky radio over the supplied crossover lead in either direction. This PR swaps the in/out pins so that, like the rest, it takes input on the tip and sends output on the ring. That means the same supplied lead and the same Crossover/Standard setting work on every Flysky radio. The schematic labels the trainer signals one way on the MCU sheet and the other way on the jack sheet, which is likely where the mix-up came from.
• B&W fix (separate commit): this is somewhat unrelated, but it's in the same area. chore: improve translation string management #6502 defined TR_JACK_MODES from TR_COUNTRY_CODES, so on radios with a headphone/trainer jack switch (X-Lite S/Pro) the jack mode choice showed country codes instead of modes.

Behaviour change

• Upgrades default to Crossover. That keeps existing behaviour on every Flysky radio except ST16.
• ST16 now matches the other Flysky radios. With the supplied crossover lead it pairs with them in both directions, confirmed on the bench with PA01. Users who used a mono lead with a tip-only radio need to select Standard, the same as on any other Flysky radio.

Testing (Flysky radios on PR firmware, Pocket on stock firmware)

Pair	Setting	Lead	Result
PL18U ↔ PA01	Crossover	supplied crossover	✅ both directions
PL18U ↔ PA01	Standard	mono	✅ both directions
PL18U ↔ PA01	Standard	straight stereo	✅ both directions
EL18 ↔ PA01	Crossover	supplied crossover	✅ both directions
PA01 ↔ RM Pocket	Standard	straight stereo	✅ both directions
PA01 ↔ RM Pocket	Standard	mono	✅ both directions
EL18 ↔ RM Pocket	Standard	mono	✅ both directions
ST16 ↔ PA01	Crossover	supplied crossover	✅ both directions

• Live switching: switching Crossover ↔ Standard on PA01 while running as slave works without a reboot.
• Upgrade: flashing over older firmware defaults to Crossover, and Standard survives a reboot.
• Companion: the option shows for Flysky boards and is hidden for TX16S. Read from and write to the radio both work.
• Not tested on hardware: NV14, NB4P, PL18 and PL18EV. They use the same jack wiring as PL18U and EL18.
• Translations: the new strings are English in every language for now.

Backport notes (2.12)

Not proposed for backport yet. A trial cherry-pick onto 2.12 shows it needs hand-porting, because 2.12 has no hw_defs JSON:

• Firmware:
  • Put TRAINER_JACK_CROSSOVER in targets/pl18/hal.h (one shared trainer block for PL18, PL18EV, PL18U, NB4P, NV14 and EL18), pa01/hal.h and st16/hal.h.
  • Drop the hw_defs JSON, models.py and schema changes.
  • trainerTipOnly:1 fits the COLORLCD spare bits there (spare:3).
  • Regenerate yaml_datastructs_*.cpp, and re-apply the strings to 2.12's i18n files.
• ST16: swap the pin defines in targets/st16/hal.h.
• Companion: 2.12 builds its hwdefs JSON from hal.h, so HasTrainerJackCrossover would need a hardcoded board check in boards.cpp, like HasAudioMuteGPIO.
• B&W fix: cherry-picks cleanly. The chore: improve translation string management #6502 bug is also in 2.12.
• Release notes: a new radio.yml key in a patch release, plus the ST16 behaviour change for upgraders.
• Testing: a 2.12 build would need re-testing on a PL18-family radio, ST16 and Companion.

Supersedes #6970. Thanks @NanoNM for tracking down the original problem and for the first fix.

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