refactor: make CodingRate a plain u8 denominator (5-8) on the wire

CodingRate was an enum with repr(u8) values that didn't match postcard's
wire encoding (variant index). Instead of papering over the mismatch,
make cr a plain u8 in RadioConfig. Host sends the denominator directly:
5 for CR 4/5, 6 for CR 4/6, etc. No lookup tables, no index confusion.

Remove CodingRate enum, cr_denom() helper, and the Bandwidth import that
was only needed for cr_denom. ErrorCode repr values also fixed to 0-based.

tools/meshcore_rx.py

@@ -914,7 +914,7 @@ RADIO_CONFIG = {
     "freq_hz": 910_525_000,
     "bw": 6,  # Khz62 = variant index 6
     "sf": 7,
-    "cr": 0,  # Cr4_5 = variant index 0
+    "cr": 5,  # CR 4/5 — denominator value
     "sync_word": 0x3444,
     "tx_power_dbm": 14,
 }