radiolib: bring LinuxSX1262 to parity with CustomSX1262

[mmmorks]

Staged on the fork for review. Final destination: l5yth/meshcore-linux linux. Base here is pr/02-config-validation so the diff shows only this change.

Summary

LinuxSX1262 carried a hand-copied subset of CustomSX1262 (the RX watchdog, the millis setters, the boosted-gain readback) and had drifted from it. This derives it from CustomSX1262 instead, keeps only the genuinely Linux-specific part (a std_init() that reads its settings from meshcored.ini at runtime), and closes the two gaps that drift had opened: two SX126x build-flag knobs with no Linux equivalent, and an AGC reset that was a bare sleep() on Linux.

What changed

• src/helpers/radiolib/LinuxSX1262.h: class LinuxSX1262 : public CustomSX1262. Upstream's RX-IRQ-timeout state machine (bounding a latched PREAMBLE_DETECTED that never becomes a packet) now applies on Linux too; before this, isReceiving() here could report busy forever after a preamble that went nowhere, deferring every transmit until getCADFailMaxDuration(). std_init() shadows the non-virtual base and takes LinuxConfig by reference.
• Two new meshcored.ini keys, mirroring the compile-time flags the MCU variants use, defaulting to the compile-time defaults (DC-DC, no patch) so existing installs are unaffected:
  • use_regulator_ldo (SX126X_USE_REGULATOR_LDO, begin()'s useRegulatorLDO)
  • rx_register_patch (SX126X_REGISTER_PATCH, bit 0 of register 0x8B5)
• LinuxSX1262Wrapper:
  • doResetAGC() override running the full SX126x reset (warm sleep, Calibrate(0x7F), calibrateImage() for the configured band), like every other SX126x board. Without it, set agc_int fell through to RadioLibWrapper's bare sleep() fallback.
  • powerOff() for parity with CustomSX1262Wrapper (cold sleep). Nothing calls it on Linux today.
  • The seventeen ((LinuxSX1262 *)_radio)-> casts become one r() accessor.
• src/helpers/radiolib/SX126xReset.h: recalibration drops DIO2-as-RF-switch, RX boosted gain and the 0x8B5 patch, and the existing sx126xResetAGC(radio, bool) restores them from SX126X_* build flags. A runtime-configured target has none of those, so this adds a second overload taking an SX126xRxSettings struct, factors the recalibration and the settings re-application into sx126xRecalibrate() / sx126xApplyRxSettings(), and replaces the three verbatim copies of the register poke with sx126xApplyRegisterPatch(). The MCU overload keeps its #ifdef path and behaviour.
• CustomSX1262.h: uses sx126xApplyRegisterPatch() (no behaviour change).
• README: settings-table rows for the two keys; the "Upstream-sync fragility" Known Gap is removed, since the wrapper no longer re-implements the interface by hand.

Why

Everything in the copied code is shared SX126x logic that upstream keeps fixing (the RX timeout handling, which IRQ flag a timeout clears, the setters themselves all changed this year). Keeping a fork of it meant re-applying each fix by hand and silently missing the ones nobody noticed; the missed RX-timeout port above is the concrete example.

How it was tested

• Clean linux_repeater build for arm64 in the container (build-docker.sh).
• Native test suite unchanged and passing.
• Not exercised on hardware: neither new key (needs a module that wants LDO or the patch) nor the reset path itself.

Dependencies

Stacked on pr/02-config-validation (uses its parse_bool() for the two new keys). Review the last commit only until that merges.

Shared code touched

• src/helpers/radiolib/SX126xReset.h (additive: struct, two helpers, a second sx126xResetAGC overload; the existing overload's behaviour is unchanged)
• src/helpers/radiolib/CustomSX1262.h (register poke replaced by the helper; no behaviour change)
• src/helpers/radiolib/LinuxSX1262.h, LinuxSX1262Wrapper.h (Linux-only)

Noticed in passing, not changed here: CustomLLCC68Wrapper.h calls sx126xResetAGC((SX126x *)_radio) with one argument, which does not match either overload's signature. That is pre-existing in this tree.