conformance: remove the local debug proxy scaffolding

[iainmcgin]

Fixes #248

Removes the local debugging scaffolding from the conformance server: start_caddy_proxy, start_tcplog_proxy, and the ENABLE_CADDY_PROXY / ENABLE_TCPLOG dispatch in main. Net 114 lines deleted.

Both helpers spawned binaries that are not part of this repository, are not built by the Taskfile, and are documented nowhere — tcplog defaulted to /tmp/tcplog/target/release/tcplog, a cargo target directory that exists on one machine. Grepping the tree for tcplog, caddy, TCPLOG_BIN, CADDY_BIN, ENABLE_TCPLOG and ENABLE_CADDY_PROXY turns up nothing outside the deleted code, so nothing else in the repo, the Taskfile or CI referenced them.

They cost nothing at runtime, since both were off unless the environment variable was set. The cost was that a contributor reading the conformance harness met a hundred lines of dead-end setup pointing at paths they do not have.

main now reports the real listener port unconditionally. The _proxy_child half of the old tuple was None on every path the deleted branches did not take, so dropping the binding changes nothing about what is kept alive. std::process::Command and Stdio became unused and are removed.

No changelog fragment. conformance is publish = false and has no surface a library user consumes, and the repo's own practice matches: of the 33 commits touching only harness, CI, docs, examples or benches — including all four since changie was adopted in #195 — none carries a fragment.

Verified beyond the build: feeding a ServerCompatRequest on stdin, the server logs its listener address and writes a ServerCompatResponse whose port field decodes to that same port, and a live Connect JSON unary call against it returns a well-formed ConformancePayload. Serving is intact end to end.

One note on CI: two handler::tests element-budget tests fail on main right now, independently of this change — fixture rot from buffa 0.9.1, fixed by #239. This diff touches one file under conformance/ and does not link that crate's test binary.