fix: correct postcard encoding for u8/i8 fields (was using varint/zigzag)

ROOT CAUSE: postcard encodes u8 and i8 as raw bytes, NOT varint/zigzag.
The Python encoders used varint() for u8 and zigzag() for i8. For small
values this was silently correct (varint(7) == raw byte 7), but for
tx_power_dbm: zigzag(14) = 28, which the new validation correctly
rejected as out of range (-9..22). This was a pre-existing bug masked
by the absence of validation.

Fix all RadioConfig field encoders in both donglora.py and meshcore_rx.py.
Fix docs/PROTOCOL.md to correctly document i8 as raw byte, not zigzag.
Restore full config validation.

docs/PROTOCOL.md

@@ -26,8 +26,9 @@ Postcard is a compact binary format. The subset used by DongLoRa:
 | Type | Encoding |
 |------|----------|
 | `u8` | Raw byte |
+| `i8` | Raw byte (two's complement) |
 | `u16`, `u32` | Varint (LEB128: 7 data bits per byte, MSB = continuation) |
-| `i8`, `i16` | Zigzag then varint (`(n << 1) ^ (n >> bits-1)`) |
+| `i16`, `i32` | Zigzag then varint (`(n << 1) ^ (n >> bits-1)`) |
 | Enum variant | Varint of 0-based variant index, then variant fields |
 | `Option<T>` | `0x00` = None, `0x01` + T = Some |
 | `Vec<u8, N>` | Varint length, then raw bytes |

examples/donglora.py

@@ -12,6 +12,7 @@ See docs/PROTOCOL.md for the full specification.
 import glob
 import struct
 import subprocess
+
 import serial
 import time
 from cobs import cobs
@@ -112,13 +113,17 @@ def decode_zigzag(data: bytes) -> tuple[int, bytes]:
 # ── Command encoding ─────────────────────────────────────────────
 
 def encode_config(cfg: dict) -> bytes:
-    """Encode a RadioConfig dict to postcard bytes."""
-    out = varint(cfg["freq_hz"])
-    out += varint(cfg["bw"])
-    out += varint(cfg["sf"])
-    out += varint(cfg["cr"])
-    out += varint(cfg["sync_word"])
-    out += zigzag(cfg["tx_power_dbm"])
+    """Encode a RadioConfig dict to postcard bytes.
+
+    Postcard wire format: u8/i8 are raw bytes, u16/u32 are varint,
+    i16/i32 are zigzag+varint, enums are varint variant index.
+    """
+    out = varint(cfg["freq_hz"])              # u32: varint
+    out += varint(cfg["bw"])                  # enum: varint variant index
+    out += struct.pack("B", cfg["sf"])        # u8: raw byte
+    out += struct.pack("B", cfg["cr"])        # u8: raw byte
+    out += varint(cfg["sync_word"])           # u16: varint
+    out += struct.pack("b", cfg["tx_power_dbm"])  # i8: raw signed byte
     return out
 
 

examples/meshcore/meshcore_rx.py

@@ -426,12 +426,12 @@ def zigzag(n: int) -> bytes:
 
 
 def encode_radio_config(cfg: dict) -> bytes:
-    out = varint(cfg["freq_hz"])
-    out += varint(cfg["bw"])
-    out += varint(cfg["sf"])
-    out += varint(cfg["cr"])
-    out += varint(cfg["sync_word"])
-    out += zigzag(cfg["tx_power_dbm"])
+    out = varint(cfg["freq_hz"])     # u32: varint
+    out += varint(cfg["bw"])         # enum: varint variant index
+    out += struct.pack("B", cfg["sf"])  # u8: raw byte
+    out += struct.pack("B", cfg["cr"])  # u8: raw byte
+    out += varint(cfg["sync_word"])  # u16: varint
+    out += struct.pack("b", cfg["tx_power_dbm"])  # i8: raw byte (signed)
     return out
 
 
@@ -931,7 +931,8 @@ def _format_rssi_snr(rssi: int, snr: int) -> str:
 
 
 def configure_and_listen(ser: serial.Serial):
-    send_cmd(ser, {"type": "SetConfig", "config": RADIO_CONFIG}, "SetConfig 910.525/62.5k/SF7/CR4_5")
+    send_cmd(ser, {"type": "Ping"}, "Ping")
+    send_cmd(ser, {"type": "SetConfig", "config": RADIO_CONFIG}, "SetConfig 910.525/62.5k/SF7/CR4/5")
     send_cmd(ser, {"type": "StartRx"}, "StartRx")
 
     print(f"\n{BWHT}Listening for packets{RST} {DIM}(Ctrl+C to stop){RST}\n")