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/external/external.ld

@@ -108,6 +108,7 @@ MEMORY
     ram_external_app_p25_tx                (rwx) : org = 0xAE030000, len = 32k
     ram_external_app_two_tone_pager        (rwx) : org = 0xAE040000, len = 32k
     ram_external_app_two_tone_rx           (rwx) : org = 0xAE050000, len = 32k
+    ram_external_app_flex_tx               (rwx) : org = 0xAE060000, len = 32k
 }
 
 SECTIONS
@@ -621,4 +622,10 @@ SECTIONS
         KEEP(*(.external_app.app_two_tone_rx.application_information));
         *(*ui*external_app*two_tone_rx*);
     } > ram_external_app_two_tone_rx
+
+    .external_app_flex_tx : ALIGN(4) SUBALIGN(4)
+    {
+        KEEP(*(.external_app.app_flex_tx.application_information));
+        *(*ui*external_app*flex_tx*);
+    } > ram_external_app_flex_tx
 }