Fix 5 bugs found via wrk stress testing the HTTP proxy

1. tcp_write infinite loop: When the output byte queue enters an error
   state (buffer full), tcp_write would loop forever because
   byte_queue_write_buf returns {NULL,0} without advancing. Break out
   of the loop when the write buffer is empty.

2. Proxy ignores async return values: toastyfs_async_get/put/delete
   can fail (return -1) but the proxy unconditionally set the operation
   to STARTED, causing it to wait forever for a result that never comes.
   Now checks return values and responds with 500 on failure.

3. Use-after-free on closed connections: When a client disconnects while
   its request is queued as PENDING, the HTTP_Conn is freed but the proxy
   still holds stale pointers. Added http_response_builder_is_valid() to
   detect and discard operations for dead connections.

4. Wrong HTTP status codes: The proxy returned 500 for all errors. Now
   returns 507 (Insufficient Storage) for FULL, 404 for NOT_FOUND, and
   503 (Service Unavailable) when the operation queue is full.

5. TCP connection leak (root cause of proxy hang): When a peer closes a
   connection, process_conn_events closes the socket and sets HUP, but
   tcp_conn_free_maybe immediately frees the TCP_Conn before
   tcp_next_event can deliver the HUP event. The HTTP server never learns
   about the disconnect, so HTTP_Conn entries accumulate until max_conns
   is reached and the proxy stops accepting new connections. Fixed by
   only calling tcp_conn_free_maybe when the CLOSED flag is already set
   (meaning the user called tcp_close), preserving the connection for
   HUP event delivery otherwise.

Also adds wrk-based stress test suite (stress-test.sh + lua scripts).

https://claude.ai/code/session_01EHoWWnVgyCFxah5WaNV4kS

lib/http_server.h

@@ -60,6 +60,7 @@ typedef struct {
 bool http_server_next_request(HTTP_Server *server,
     HTTP_Request **request, HTTP_ResponseBuilder *builder);
 
+bool http_response_builder_is_valid(HTTP_ResponseBuilder builder);
 void http_response_builder_status(HTTP_ResponseBuilder builder, int status);
 void http_response_builder_header(HTTP_ResponseBuilder builder, string header);
 void http_response_builder_content(HTTP_ResponseBuilder builder, string content);