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 |