Add MDC-1200 TX external app (#3083)

Implements MDC-1200 single and double-packet TX as a PPMA external app.

Features:
- Supports PTT ID, Emergency, Radio Check, Stun/Unstun, Call Alert,
  Selective Call (1 & 2), Status Request/Response, Remote Monitor, Message
- Auto-fills Arg byte on Op selection
- AFSK FSK modulation: 1800 Hz mark / 1200 Hz space, 1200 baud
- NRZI encoding with Golay FEC per MDC-1200 spec
- Tap-to-edit frequency via keypad
- Default TX gain of 30 dB
- Uses AFSK baseband (same as POCSAG/SAME TX)
- All buffers kept off the stack to avoid hard faults in ext app environment

* mdc_tx: address PR review feedback

- Move MDC helpers into ui::external_app::mdc_tx namespace (drop anonymous namespace)
- Use AFSK_TX_SAMPLERATE macro instead of hardcoded 1536000
- Add #include "modems.hpp" explicitly for AFSK_TX_SAMPLERATE
- Replace n*8/n*16 pixel positions with UI_POS_X/UI_POS_Y macros
- Fix field comments: 'hex bytes' -> 'decimal bytes (0-255)'
- Fix TransmitterView frequency_step: 1750000 -> 12500 (12.5kHz NFM step)
- Fix MDC preamble: replace leading 0x00 bytes with 0xAA
  after NRZI encoding stayed 0x00, terminating TX on the first word.
  0xAA bytes NRZI-encode to 0xFF (continuous 1800Hz mark tone), which
  is the standard MDC-1200 preamble and does not trigger the sentinel.

firmware/application/external/external.cmake

@@ -107,6 +107,10 @@ set(EXTCPPSRC
 	external/same_tx/main.cpp
 	external/same_tx/ui_same_tx.cpp
 
+	#mdc_tx
+	external/mdc_tx/main.cpp
+	external/mdc_tx/ui_mdc_tx.cpp
+
 	#sstvrx
 	external/sstvrx/main.cpp
 	external/sstvrx/ui_sstvrx.cpp
@@ -360,6 +364,7 @@ set(EXTAPPLIST
 	#morse_tx
 	sstvtx
 	same_tx
+	mdc_tx
 	sstvrx
 	random_password
 	acars_rx