Implement keep-alive / Connection: close / HTTP-1.0 negotiation

[harmon25]

Copilot review (PR #4, commit 489abc8, src/httpd.erl:329-331).

The code explicitly defers proper connection negotiation:

%% NOTE: HTTP/1.0 default-close and Connection: close semantics are not yet
%% implemented; that negotiation is deferred to a follow-up.  Handlers that
%% need to force a close should return {close, ...} instead.

{reply, ...} responses always keep the connection open regardless of the request's HTTP version or a Connection: close header. To be spec-compliant the server should:

• close after the response for HTTP/1.0 unless Connection: keep-alive,
• close when the client sends Connection: close,
• otherwise keep-alive for HTTP/1.1.

The version field and lowercase header normalization are already in place, so the inputs needed for an is_keep_alive/1 helper exist.

Validated against current code: deferred / not yet implemented.