Implement generated packet wire codecs (#47)

codegen/README.md

@@ -51,6 +51,17 @@ receive the same zero defaults as the golden generator. The generated `Length`
 metadata intentionally preserves the golden generator's variable-block count
 semantics.
 
+The same output contains the native wire implementation for every generated
+packet and block. It preserves high-, medium-, and low-frequency headers,
+network-order packet IDs, sequence numbers and appended ACKs; the reference
+field endianness and widths; one- and two-byte variable prefixes; fixed-field
+padding and truncation; fixed and variable block counts; normalized quaternion
+encoding; zerocoding boundaries; and the reference `ToBytesMultiple` splitting
+rules. Readers use checked bounds and allocation arithmetic and reject unknown
+IDs or truncated payloads without panicking. Generated tests round-trip all 483
+packet types and exercise every truncation boundary, while focused fixtures
+assert exact reference bytes and malformed-input behavior.
+
 Generation also compares all packet, block, field, field-type, and `PacketType`
 entries against the pinned compiled public API catalog. This rejects a template
 parser or naming change even if the emitted Rust would otherwise compile. The