Nrf promicro hf (#11966)

* define txen pin to allow sx128x testing

* nrf52 promicro: gate the P1.06 TXEN pin, and add an optional SX1280 build

P1.06 was wired to SX126X_TXEN unconditionally earlier in this series, to drive
an external PA on a bench unit. That is not safe to ship: three envs share this
variant.h, and P1.06 is PIN_WIRE_SCL on the EasyProMicro pinout and
PIN_EINK_BUSY on the InkHUD one. The default builds go back to RADIOLIB_NC.

Put it behind -D PROMICRO_SX126X_TXEN, refused wherever the pin is already
claimed. The EasyProMicro arrangement has its own SX126X_TXEN on P1.00 in the
#else branch and never reaches this, so only the e-ink exclusion is needed.

The pin was wanted for SX1280 in the first place - every other SX128X line is
already assigned on this board and TXEN was the one missing - so add that too,
behind -D PROMICRO_SX1280 and a new nrf52_promicro_diy-sx1280 env. Both opt-ins
claim P1.06, so asking for both is an #error rather than a silent pick.

Costs about 10.7 KB: the image ends at 0xE5140, 19 KB clear of the warm region,
against 0xE2710 and 30 KB for the base env.

* nrf52 promicro: keep P1.06 off TXEN on a MESHTASTIC_USE_EINK_UI build

configuration.h includes variant.h before it derives USE_EINK from
MESHTASTIC_USE_EINK_UI, so a build enabling the e-ink UI that way passed
the guard and could give the panel's BUSY line to TXEN. Test both.

---------

Co-authored-by: Jonathan Bennett <jbennett@incomsystems.biz>

variants/nrf52840/diy/nrf52_promicro_diy_tcxo/variant.h

@@ -183,7 +183,9 @@ https://github.com/brad112358/easy_E22
 
 // P1.06 is the only free pin left for a TXEN line, but it is also the e-ink panel's BUSY line.
 // EasyProMicro, where it is SCL, takes the #else branch below and has its own TXEN on P1.00.
-#if !defined(MESHTASTIC_INCLUDE_NICHE_GRAPHICS) && !defined(USE_EINK)
+#if !defined(MESHTASTIC_INCLUDE_NICHE_GRAPHICS) && !defined(USE_EINK) &&                                                         \
+    !(defined(MESHTASTIC_USE_EINK_UI) && MESHTASTIC_USE_EINK_UI)
+
 #define PROMICRO_P106_FREE_FOR_TXEN
 #endif