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>

docs/uconsole.md

@@ -274,7 +274,6 @@ en_pins = [27]   # the AIO v2 needs this
 | `en_pins` | `[]` | Power-enable lines to raise before the chip is touched. The **AIO v2 needs `[27]`.** |
 | `use_dio2_rf`, `use_dio3_tcxo` | `true`, `true` | Whether DIO2 drives the RF switch and DIO3 a TCXO (both on for the AIO). |
 | `is_waveshare` | `false` | The Waveshare HAT's wiring quirks, which the radio library knows about. |
-| `preamble_length` | `12` | LoRa preamble symbols. |
 | `python` | `""` (empty) | An interpreter to run the node under, when the one MeshTerm would find on its own isn't the one you want. Empty lets MeshTerm look. |
 
 An unknown key, or one of the wrong type, stops MeshTerm at startup with the profile
@@ -561,7 +560,7 @@ Then `systemctl --user restart meshterm-spi-bridge.service`.
 | `MESHCORE_SPREADING_FACTOR`, `MESHCORE_BANDWIDTH`, `MESHCORE_CODING_RATE` | the modem preset — all three must match the mesh you are joining. Whole numbers only: bandwidth in Hz (`62500`), coding rate as the denominator (`5` for 4/5). A decimal or `4/5` stops the bridge at startup. |
 | `MESHCORE_TXEN_PIN`, `MESHCORE_RXEN_PIN`, `MESHCORE_EN_PINS` | the RF-switch and power-enable lines a board may need (`-1` for none; `EN_PINS` is a comma list — the AIO v2 wants `27`) |
 | `MESHCORE_USE_DIO2_RF`, `MESHCORE_USE_DIO3_TCXO`, `MESHCORE_IS_WAVESHARE` | whether DIO2 drives the RF switch and DIO3 the TCXO (both on for the AIO), and a flag for a different vendor's wiring the runtime knows about |
-| `MESHCORE_GPIO_CHIP`, `MESHCORE_USE_GPIOD_BACKEND`, `MESHCORE_PREAMBLE_LENGTH` | which gpiochip, whether to drive it through `gpiod`, and the LoRa preamble |
+| `MESHCORE_GPIO_CHIP`, `MESHCORE_USE_GPIOD_BACKEND`, `MESHCORE_PREAMBLE_LENGTH` | which gpiochip, whether to drive it through `gpiod`, and the LoRa preamble (leave it unset: the bridge follows MeshCore, 32 symbols up to SF8 and 16 above, and a shorter one leaves the radio deaf to most of the mesh) |
 | `MESHTERM_PYMC_PYTHON` | force a specific interpreter instead of letting the bridge discover one |
 
 The bridge passes a knob only when the runtime's radio constructor accepts it, so the same