FLEX RX: comprehensive decoder rewrite (#3131)

Baseband (proc_flex.cpp):
- Fix numeric vector field width (3-bit n+1 for types 3,4,7)
- Fix capcode range classification for long address components
- Add long address decode (Set 1-2, 1-3/1-4, 2-3)
- Add vector checksum validation (4-bit nibble sum)
- Add tone-only boundary detection via vector pre-scan
- Add idle phase detection (skip all-zeros/all-ones before BCH)
- Skip spurious TON for capcode 1 (BCH-corrected idle artifact)
- Add BIW parsing: SSID1/SSID2, DATE, TIME, TZ, SYSMSG, CHAN
- Add alpha fragment flags: frag, more_frag, seq, new, maildrop, sig
- Add secure message enc= field (alpha/binary/separate)
- Add numbered numeric flags: seq, new, fmt
- Add HEX/Binary header decode: block_bits, new, maildrop, rtl
- Add short instruction decode: temp group slot/target, sys events
- Add short message (SMSG) decode: tone/numeric/source/numbered
- Add group/temp-group/priority flags per address
- Add S2 C-pattern detection with ±1 symbol boundary correction
- Extract FIW roaming, repeat, traffic fields
- Replace snprintf with lightweight str_* helpers (no heap/_sbrk)
- Mark uncorrectable BCH words as ? instead of aborting phase
- ETX/NUL handling: trim trailing padding, show mid-message as ?

App (ui_flex_rx.cpp):
- Add status bar: cycle/frame, bitrate, polarity, time, timezone
- Add network info bar: LID, CZ, CC, roaming flag
- Pipe-delimited serial output for all message types
- BIW events to serial and status bar (not console)
- Human-readable console: +GRP for temp group, G/TG/P flags
- USB serial via UsbSerialAsyncmsg

Packet (flex_defs.hpp):
- uint64_t capcode (long address support)
- 256-byte message buffer
- Fragment flags, BIW raw values, address type, group/priority

Known limitation: phase label unreliable on multi-phase modes
(3200/2FSK, 6400/4FSK). Data always decodes correctly.

firmware/baseband/proc_flex.hpp

@@ -66,6 +66,11 @@ struct FlexStateInfo {
     unsigned int fiwcount = 0;
     State Current = State::SYNC1;
     State Previous = State::SYNC1;
+
+    // S2 C-pattern detection
+    uint16_t sync2_shiftreg = 0;  // 16-bit shift register for C match
+    int sync2_c_pos = -1;         // symbol position where C was found (-1=not found)
+    int sync2_cinv_pos = -1;      // symbol position where inv.C was found
 };
 
 struct FlexSync {
@@ -81,7 +86,9 @@ struct FlexFIW {
     unsigned int checksum = 0;
     unsigned int cycleno = 0;
     unsigned int frameno = 0;
-    unsigned int fix3 = 0;
+    unsigned int roaming = 0;  // bit 15: n (1=roaming provided)
+    unsigned int repeat = 0;   // bit 16: r (1=multiple transmission)
+    unsigned int traffic = 0;  // bits 17-20: t3-t0
 };
 
 struct FlexPhase {
@@ -98,10 +105,25 @@ struct FlexData {
     FlexPhase PhaseD;
 };
 
+enum class AddrType : uint8_t {
+    SHORT,      // normal individual
+    LONG,       // 9-10 digit
+    TEMPORARY,  // 0x1F7800-0F (16 group slots)
+    OPERATOR,   // 0x1F7810-1F (system messages)
+    NETWORK,    // 0x1F6800-77FF (NID)
+    INFO_SVC,   // 0x1F2800-67FF (under study)
+    RESERVED,   // reserved ranges
+    UNKNOWN
+};
+
 struct FlexDecode {
     PageType type = PageType::ALPHANUMERIC;
     int long_address = 0;
     int64_t capcode = 0;
+    AddrType addr_type = AddrType::SHORT;
+    int is_group = 0;
+    int is_temp_group = 0;
+    int is_priority = 0;
 };
 
 }  // namespace flex
@@ -161,7 +183,7 @@ class FlexProcessor : public BasebandProcessor {
 
     // Parsing
     void parse_capcode(uint32_t aw1);
-    void parse_alphanumeric(uint32_t* phaseptr, char PhaseNo, int mw1, int mw2, int flex_groupmessage);
+    void parse_alphanumeric(uint32_t* phaseptr, const uint8_t* word_bad, char PhaseNo, int mw1, int mw2, int flex_groupmessage);
     void parse_numeric(uint32_t* phaseptr, char PhaseNo, int j);
     void parse_tone_only(uint32_t* phaseptr, char PhaseNo, int j);
     void parse_unknown(uint32_t* phaseptr, char PhaseNo, int mw1, int mw2);