fix(http1): recognize \n\r\n as a head terminator in the partial-read fast path

[nam2ee]

Closes #4145.

is_complete_fast recognizes \r\n\r\n and \n\n as head terminators but not \n\r\n, while the full parser (httparse) accepts all three. So a request whose head ends with \n\r\n parses fine when it arrives in a single read, but stalls when it arrives split across reads: the fast path never reports the head complete and the connection keeps waiting for more bytes.

This extends the \n branch to also accept a following \r\n, using the same panic-safe slicing idiom as the \r branch. Added the \n\r\n witness and the \n\r negative to test_is_complete_fast, plus a parse-level test documenting that the full parser accepts this terminator.