In http_get, when the server closes the TCP connection before the HTTP
header block completes (n == 0 on the byte-by-byte header read), or sends
a header block exceeding 64 KiB, the code returned KeydbParse (E8004).
Both are connection/protocol-level faults from the server, not parse
failures of keydb content — the keydb bytes were never received. Return
KeydbConnect (E8000) instead, which already covers TCP-level exchange
failures. A CLI user hitting a transient drop or a redirect target that
immediately closes now sees the correct 'server hung up' diagnostic
rather than 'the downloaded file was malformed'.
Add a regression test that stands up a loopback listener which accepts
then drops the connection before headers, asserting KeydbConnect.
Matthew Jackson
e52689579b