use an unsigned accumulator in base64_encode

[metsw24-max]

base64_encode folds each input byte into a signed int and shifts it left by eight every round; once four bytes are in, the top bit is set and the next shift left-shifts a negative int, which is undefined behaviour (ubsan reports a left shift of a negative value, and the same shift recurs in the trailing six-bit fixup). the helper runs on the server's websocket accept-key path over the sha1 digest and in the basic-auth header builder, so it is reachable from request handling. making the accumulator a uint32_t removes the ub and keeps the output byte-for-byte identical, since every emitted six-bit group is masked with 0x3f and is independent of the accumulator's signedness. the added test pins the rfc 4648 vectors plus a high-byte input that keeps the top bit set across rounds.

[yhirose]

Looks like there are quite a few errors...

[metsw24-max]

Those were all the same failure: the new test calls detail::base64_encode, but in the split build split.py strips inline and moves the definition into httplib.cc, so it isn't declared in the public header. I've forward-declared it in the test the same way getaddrinfo_with_timeout already is, so it resolves in both the header-only and split builds. Confirmed both compile and the test passes locally.