encode_chunked() split the payload into shards first, then overwrote
the first 4 bytes of shard 0 with a u32 length header, then re-ran
Reed-Solomon to regenerate parity over the now-corrupted shards. The
decoder correctly read the length header and trimmed `[4..4+len]`
from the reconstructed buffer, but those first 4 bytes had already
been destroyed on the encode side, so every chunked mesh payload
lost its first 4 bytes.
Restructure: reserve 4 bytes for the length header up front, build
a single contiguous [len][data][pad] buffer, then split into shards.
Parity is computed over the correct shards on the first pass, no
double-encode needed.
Update test_chunk_roundtrip_medium: 500 bytes + 4-byte header = 504
bytes, which is 5 data shards (ceil(504/124)), not 4. The old test
assertion was wrong all along and masked the corruption bug because
it only checked the roundtripped bytes, which is exactly what we
need to verify. New assertion is correct.
Verified: all 7 transport::chunking tests pass.
f6efe2f356