Move HTTP from cowboy to livery

[benoitc]

Serves HTTP through livery 0.7.0 instead of cowboy, and drives the test suites with livery_client instead of hackney.

What changes

• The listener is owned by a new supervised hornbeam_listener; hornbeam:info/0 and hornbeam:is_running/0 replace ranch introspection.
• hornbeam_handler is a livery handler. WSGI and ASGI responses resolve through livery_resp:stream_deferred/1 in the per-request process, so request-body chunks and Python events share one mailbox.
• WebSocket runs on the ws_handler behaviour (new hornbeam_ws_handler); hornbeam_websocket keeps the upgrade entry point, scope building, and the session registry.
• Duplicate request headers are preserved in the ASGI scope and joined with ", " in WSGI HTTP_* keys.
• New max_body option (default infinity) caps the request body at the listener.

Breaking

• Route shape: routes are livery router entries {Method | '_', Pattern, HandlerFun | {Mod, Fun}} instead of cowboy {Path, HandlerModule, Opts} tuples. Cowboy handler modules are rejected with {error, {invalid_route, _}}.
• Inactive config keys: max_request_line_size, max_header_size and max_headers are still accepted but do nothing (not exposed by livery).
• hornbeam_wsgi removed: the classic build_environ/1,2 path was dead code; the live path is hornbeam_request:build_wsgi_tuple/2.

Requires livery 0.7.0

Five behaviours depend on the 0.7.0 release: livery_req:inform/3 for 103 Early Hints, the peer on plain HTTP/1.1 requests (WSGI REMOTE_ADDR, ASGI client), synchronous connection teardown in livery:stop_service/1, max_frame_size/compress on livery_ws:upgrade/3, and {remote, Code, Reason} in the ws handler's terminate/2.

Also fixes the /early-hints route in the ASGI test app, which sent status 103 through http.response.start (treated as the final head) rather than http.response.informational, so it had never worked.

Known limitations

• TLS serves HTTP/1.1 only; cowboy's start_tls negotiated HTTP/2 via ALPN. Wiring livery's https (h2) listener is a follow-up.
• 103 Early Hints go out on HTTP/1.1 and HTTP/2 but not HTTP/3, which has no interim responses in livery yet. The http.response.early_hints ASGI extension is advertised accordingly.