SIMD @Vector internet checksum proven byte-equivalent to scalar across
every length 0..256 plus the 0xb861 IPv4 KAT. RFC 1624 incremental
update proven against the RFC section 4 worked example
(0xDD2F/0x5555/0x3285 -> 0x0000) plus 4096 random full-recompute trials.
Reusable TCP pseudo-header checksum helper; smoke.zig now reuses it.
New cookie.zig: stateless SipHash64(2,4) SYN-cookie over the 4-tuple,
full-128-bit randomSecure key, u64 generate with write-site u32
truncation, ack == cookie +% 1 wrapping validation. Reproduces the
published SipHash reference vector plus a pinned golden cookie KAT.
All green Debug + ReleaseSafe (14/14). Version bumped to 0.0.0-m1.
e6e6703aba