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.

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."""
+    """Encode a RadioConfig dict to postcard bytes.
-    out = varint(cfg["freq_hz"])
+
-    out += varint(cfg["bw"])
+    Postcard wire format: u8/i8 are raw bytes, u16/u32 are varint,
-    out += varint(cfg["sf"])
+    i16/i32 are zigzag+varint, enums are varint variant index.
-    out += varint(cfg["cr"])
+    """
-    out += varint(cfg["sync_word"])
+    out = varint(cfg["freq_hz"])              # u32: varint
-    out += zigzag(cfg["tx_power_dbm"])
+    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