The SPI radio listens for the preamble the mesh actually sends

MeshCore transmits a 32-symbol preamble up to SF8 (16 above), but the
node and the bridge configured the SX1262 for the library's fixed 12.
The chip waits for the sync word only about as long as the preamble it
expects, so it locked onto each packet, gave up, and locked on again
further along the same preamble - decoding a packet only when it
happened to catch the tail. On the uConsole that meant 0 of 5 detected
packets decoded after a send; with 32 symbols, 4 of 4.

The node now derives the preamble from the spreading factor as MeshCore
does, and re-sends it to the listening chip when Device config changes
the SF. preamble_length leaves the SPI wiring table (it was protocol,
never wiring) and is retired; the bridge defaults to the same rule.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

scripts/uconsole/meshterm-spi-bridge

@@ -1043,6 +1043,11 @@ def run_bridge(host: str, port: int, node_name: str) -> int:
         hw = load_hardware_config_from_env()
         log.info("radio config %s", hw.to_log_string())
         radio = create_radio(SX1262Radio, hw, log.info)
+        # The same preamble rule as the direct path below: unless the environment
+        # names one, follow MeshCore (32 symbols up to SF8, 16 above) so the radio
+        # hears the mesh; begin() reads it, so this is in time.
+        if "MESHCORE_PREAMBLE_LENGTH" not in os.environ and hasattr(radio, "preamble_length"):
+            radio.preamble_length = 32 if hw.spreading_factor <= 8 else 16
         key_path = identity_key_path()
         if key_path.exists():
             identity = LocalIdentity(key_path.read_bytes())
@@ -1094,7 +1099,14 @@ def run_bridge(host: str, port: int, node_name: str) -> int:
             "spreading_factor": _int("MESHCORE_SPREADING_FACTOR", 7),
             "bandwidth": _int("MESHCORE_BANDWIDTH", 62_500),
             "coding_rate": _int("MESHCORE_CODING_RATE", 5),
-            "preamble_length": _int("MESHCORE_PREAMBLE_LENGTH", 12),
+            # MeshCore's own rule (RadioLibWrapper::preambleLengthForSF): 32 symbols up
+            # to SF8, 16 above. The receiver must match it, not just the sender — the
+            # SX1262 waits for the sync word only about as long as the preamble it expects,
+            # so the old fixed 12 against a mesh sending 32 decoded almost nothing.
+            "preamble_length": _int(
+                "MESHCORE_PREAMBLE_LENGTH",
+                32 if _int("MESHCORE_SPREADING_FACTOR", 7) <= 8 else 16,
+            ),
             "is_waveshare": _bool("MESHCORE_IS_WAVESHARE", False),
             "use_dio2_rf": _bool("MESHCORE_USE_DIO2_RF", True),
             "use_dio3_tcxo": _bool("MESHCORE_USE_DIO3_TCXO", True),