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/mdc_tx/main.cpp

@@ -0,0 +1,83 @@
+/*
+ * Copyright (C) 2025 Sarah Rose
+ *
+ * This file is part of PortaPack.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2, or (at your option)
+ * any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; see the file COPYING.  If not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#include "ui.hpp"
+#include "ui_mdc_tx.hpp"
+#include "ui_navigation.hpp"
+#include "external_app.hpp"
+
+namespace ui::external_app::mdc_tx {
+void initialize_app(ui::NavigationView& nav) {
+    nav.push<MdcTxView>();
+}
+}  // namespace ui::external_app::mdc_tx
+
+extern "C" {
+
+__attribute__((section(".external_app.app_mdc_tx.application_information"), used))
+application_information_t _application_information_mdc_tx = {
+    /*.memory_location    = */ (uint8_t*)0x00000000,
+    /*.externalAppEntry   = */ ui::external_app::mdc_tx::initialize_app,
+    /*.header_version     = */ CURRENT_HEADER_VERSION,
+    /*.app_version        = */ VERSION_MD5,
+
+    /*.app_name           = */ "MDC-1200 TX",
+    /*.bitmap_data        = */ {
+        0x00,
+        0x00,
+        0xFE,
+        0x7F,
+        0xFF,
+        0xFF,
+        0xBB,
+        0xD0,
+        0xFF,
+        0xFF,
+        0xFF,
+        0xFF,
+        0x0B,
+        0xE1,
+        0xFF,
+        0xFF,
+        0xFF,
+        0xFF,
+        0xEB,
+        0xD0,
+        0xFF,
+        0xFF,
+        0xFE,
+        0x7F,
+        0x70,
+        0x00,
+        0x30,
+        0x00,
+        0x10,
+        0x00,
+        0x00,
+        0x00,
+    },
+    /*.icon_color         = */ ui::Color::orange().v,
+    /*.menu_location      = */ app_location_t::TX,
+    /*.desired_menu_position = */ -1,
+    /*.m4_app_tag         = */ {'P', 'A', 'F', 'T'},
+    /*.m4_app_offset      = */ 0x00000000,
+};
+}