FLEX TX: pager transmitter with long address support (1600/2FSK) (#3132)

External app for encoding and transmitting FLEX pager frames at
1600bps/2FSK. Supports short (1-1,933,312) and long (2,101,249-
4,297,068,542) addresses across all encoding sets (1-2, 1-3/1-4, 2-3).

Message types:
- Alphanumeric (vector type 5): 7-bit chars, signature, K checksum
- Standard Numeric (vector type 3): 4-bit BCD, K checksum
- Short/tone-only (vector type 2): space-filled BCD in vector word
- Short numeric (vector type 2): BCD digits in vector word (3/8 digits)

Frame structure:
- FIW (Frame Information Word): cycle, frame, roaming flag
- BIW (Block Information Word) support with up to 3 extra words:
  Date, Time, SysInfo/timezone+DST, SSID1 (Simulcast System ID:
  Local ID + Coverage Zone), SSID2 (Country Code)
- BCH(31,21) encoding with bit reversal, block interleaving
- Two frames per TX: message R=1 (new) then R=0 (duplicate)
- Message number N (0-63): random seed from RTC, auto-increments
- ERS (Emergency Re-Sync) burst: configurable N x 42-cycle bursts
  with 8-cycle preamble per frame

UI:
- Capcode info line: address type, computed frame/phase (reference)
- TX status: step counter with ERS/frame progress
- Set params view: Date/Time/TZ/DST, LocID/CovZone, CountryCode,
  Roaming, message number, ERS count
- Date/time auto-populated from RTC with calendar-equivalent year
  mapping for years beyond 2025 (per ARIB STD-43A)
- Year validation with equiv-year popup for out-of-range values
- Capcode and BIW flags persisted via SettingsManager

Serial interface:
- sendflex <capcode> <type> <msglen> + payload
- Types: 0=alpha, 1=numeric, 2=short/tone, 3=short numeric
- FlexTosendMessage (ID 103) for serial-to-app communication
- App must be open to receive serial commands

firmware/application/usb_serial_shell.cpp

@@ -1353,6 +1353,53 @@ static void cmd_sendpocsag(BaseSequentialStream* chp, int argc, char* argv[]) {
     chprintf(chp, "ok\r\n");
 }
 
+static void cmd_sendflex(BaseSequentialStream* chp, int argc, char* argv[]) {
+    const char* usage = "usage: sendflex <capcode> <type> <msglen>\r\n  type: 0=alpha 1=numeric 2=short/tone 3=short numeric\r\n";
+    if (argc < 3) {
+        chprintf(chp, usage);
+        return;
+    }
+    uint64_t capcode = strtoull(argv[0], nullptr, 10);
+    if (capcode < 1 || capcode > 4297068542ULL) {
+        chprintf(chp, "error, capcode 1-4297068542\r\n");
+        return;
+    }
+    int type = atoi(argv[1]);
+    if (type < 0 || type > 3) {
+        chprintf(chp, "error, type 0=alpha 1=numeric 2=short/tone 3=short numeric\r\n");
+        return;
+    }
+    int msglen = atoi(argv[2]);
+    if (msglen < 0 || msglen > 240) {
+        chprintf(chp, "error, msglen max 240\r\n");
+        return;
+    }
+
+    uint8_t msg[240] = {0};
+    if (msglen > 0) {
+        chprintf(chp, "send %d bytes\r\n", msglen);
+        int offset = 0;
+        do {
+            size_t bytes_to_read = (msglen - offset) > USB_BULK_BUFFER_SIZE ? USB_BULK_BUFFER_SIZE : (msglen - offset);
+            size_t bytes_read = chSequentialStreamRead(chp, &msg[offset], bytes_to_read);
+            if (bytes_read != bytes_to_read)
+                return;
+            offset += bytes_read;
+        } while (offset < msglen);
+    }
+
+    auto evtd = getEventDispatcherInstance();
+    if (!evtd) return;
+    auto top_widget = evtd->getTopWidget();
+    if (!top_widget) return;
+    auto nav = static_cast<ui::SystemView*>(top_widget)->get_navigation_view();
+    if (!nav) return;
+
+    FlexTosendMessage message{capcode, (uint8_t)type, (uint8_t)msglen, msg};
+    EventDispatcher::send_message(message);
+    chprintf(chp, "ok\r\n");
+}
+
 static void cmd_asyncmsg(BaseSequentialStream* chp, int argc, char* argv[]) {
     const char* usage = "usage: asyncmsg x, x can be enable or disable\r\n";
     if (argc != 1) {
@@ -1500,6 +1547,7 @@ static const ShellCommand commands[] = {
     {"pmemreset", cmd_pmemreset},
     {"settingsreset", cmd_settingsreset},
     {"sendpocsag", cmd_sendpocsag},
+    {"sendflex", cmd_sendflex},
     {"asyncmsg", cmd_asyncmsg},
     {"setfreq", cmd_setfreq},
     {"getres", cmd_getres},