Replace io:format error logging with ?TRACE on hot paths

[harmon25]

Copilot review (PR #4, commit ea95a0a, src/httpd.erl:154).

The request error path logs unconditionally on every parse error:

A:E:S ->
    io:format("Caught error: ~p:~p:~p~n", [A, E, S]),
    {send_close, create_error(?BAD_REQUEST, E)}

This is noisy and expensive on embedded targets and violates the AGENTS.md convention ("gate noise behind ?TRACE, not io:format"). A malicious/garbage client can spam serial output.

There is a similar unconditional io:format in src/httpd_env_api_handler.erl:100.

Fix: route these through the ?TRACE macro so they can be compiled out.

Validated against current code: still present.