P4.1-001: 2s read timeout on accepted sockets (idle preconnect wedged the loop)

A browser speculative preconnection sends no bytes; the sequential
accept loop blocked in read() on it forever while real requests sat in
the backlog — LAN clients saw a dead server while curl (connect+send in
one shot) worked. SO_RCVTIMEO frees the loop. Regression case pinned in
tests/server_http.sx (fails 000 pre-fix, 200 post-fix).

src/server/distd.sx

@@ -290,6 +290,7 @@ run_server :: (store_dir: string, port: s64) -> !ServeErr {
     while true {
         client := sock.accept(fd, null, null);
         if client < 0 { continue; }
+        http.set_read_timeout(client, 2);
 
         gpa := GPA.init();
         arena := Arena.init(xx gpa, 65536);