[feat](trx-rs): add FT4 decoder support

Reuse the existing ft8_lib C library (FTX_PROTOCOL_FT4) and FT8
decoder infrastructure to add FT4 decoding across the full stack.

Changes:
- trx-ft8: add protocol param to ft8_decoder_create; add Ft8Decoder::new_ft4()
- trx-core: DecodedMessage::Ft4 variant, AUDIO_MSG_FT4_DECODE (0x14),
  ft4_decode_enabled/ft4_decode_reset_seq state, SetFt4DecodeEnabled/
  ResetFt4Decoder commands, protocol mapping
- trx-server: DecoderHistories::ft4, run_ft4_decoder (7.5s slots via
  now*2/15), run_background_ft4_decoder, history push/replay, decoder
  task spawn
- trx-frontend-http: ft4_history in FrontendRuntimeContext,
  toggle/clear endpoints, /ft4.js route, bookmark/scheduler/background
  decode support, DecodeHistoryPayload ft4 field
- web: ft4.js plugin (7.5s period timer, reuses FT8 CSS/map infra),
  FT4 subtab in index.html, app.js dispatch (onServerFt4/Batch,
  restoreFt4History), decode-history-worker HISTORY_GROUP_KEYS

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

src/decoders/trx-ft8/src/ft8_wrapper.c

@@ -108,7 +108,7 @@ typedef struct
     float freq_hz;
 } ft8_decode_result_t;
 
-ft8_decoder_t* ft8_decoder_create(int sample_rate, float f_min, float f_max, int time_osr, int freq_osr)
+ft8_decoder_t* ft8_decoder_create(int sample_rate, float f_min, float f_max, int time_osr, int freq_osr, int protocol)
 {
     ft8_decoder_t* dec = (ft8_decoder_t*)calloc(1, sizeof(ft8_decoder_t));
     if (!dec)
@@ -120,7 +120,7 @@ ft8_decoder_t* ft8_decoder_create(int sample_rate, float f_min, float f_max, int
     dec->cfg.sample_rate = sample_rate;
     dec->cfg.time_osr = time_osr;
     dec->cfg.freq_osr = freq_osr;
-    dec->cfg.protocol = FTX_PROTOCOL_FT8;
+    dec->cfg.protocol = (protocol == 0) ? FTX_PROTOCOL_FT4 : FTX_PROTOCOL_FT8;
 
     hashtable_init();
     monitor_init(&dec->mon, &dec->cfg);

src/decoders/trx-ft8/src/lib.rs

@@ -37,6 +37,7 @@ extern "C" {
         f_max: c_float,
         time_osr: c_int,
         freq_osr: c_int,
+        protocol: c_int,
     ) -> *mut c_void;
     fn ft8_decoder_free(dec: *mut c_void);
     fn ft8_decoder_block_size(dec: *const c_void) -> c_int;
@@ -69,6 +70,7 @@ impl Ft8Decoder {
                 F_MAX_HZ,
                 TIME_OSR as c_int,
                 FREQ_OSR as c_int,
+                1, // FTX_PROTOCOL_FT8
             );
             let inner = NonNull::new(ptr).ok_or_else(|| "ft8_decoder_create failed".to_string())?;
             let block_size = ft8_decoder_block_size(inner.as_ptr()) as usize;
@@ -84,6 +86,30 @@ impl Ft8Decoder {
         }
     }
 
+    pub fn new_ft4(sample_rate: u32) -> Result<Self, String> {
+        unsafe {
+            let ptr = ft8_decoder_create(
+                sample_rate as c_int,
+                F_MIN_HZ,
+                F_MAX_HZ,
+                TIME_OSR as c_int,
+                FREQ_OSR as c_int,
+                0, // FTX_PROTOCOL_FT4
+            );
+            let inner = NonNull::new(ptr).ok_or_else(|| "ft8_decoder_create failed".to_string())?;
+            let block_size = ft8_decoder_block_size(inner.as_ptr()) as usize;
+            if block_size == 0 {
+                ft8_decoder_free(inner.as_ptr());
+                return Err("invalid FT4 block size".to_string());
+            }
+            Ok(Self {
+                inner,
+                block_size,
+                sample_rate,
+            })
+        }
+    }
+
     pub fn block_size(&self) -> usize {
         self.block_size
     }