distd: read timeout 2s -> 250ms (interim, PLAN-HTTPZ)

Each idle browser preconnect blocks the sequential accept loop for the
full SO_RCVTIMEO serially (measured: ~1.9s behind one, ~3.9s behind
two), so the timeout must stay well under human patience until the
readiness loop lands (PLAN-HTTPZ A1 retires the mechanism).
set_read_timeout takes milliseconds now.

src/server/distd.sx

@@ -1052,7 +1052,7 @@ run_server :: (store_dir: string, port: i64) -> !ServeErr {
     while true {
         client := sock.accept(fd, null, null);
         if client < 0 { continue; }
-        http.set_read_timeout(client, 2);
+        http.set_read_timeout(client, 250);
 
         gpa := GPA.init();
         arena := Arena.init(xx gpa, 65536);