add RFC 8949 test vectors, fix integer overflow on malformed input
Run 778 canonical CBOR specification vectors from RFC 8949 Appendix A:
invalid vectors must be rejected, valid canonical vectors must round-trip,
valid non-canonical vectors must be rejected by DAG-CBOR.
The vectors exposed an integer overflow bug: malformed CBOR with huge
length claims (e.g., 0xFFFFFFFFFFFFFFFF byte string) caused arithmetic
overflow in pos + len instead of returning UnexpectedEof. Fixed by using
std.math.add and std.math.cast for all length arithmetic in decodeAt,
readText, readBytes, and skipValue.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>