tcl: Phase 5c TCP sockets — client + server

Three new SX primitives wrapping Unix socket APIs:
- socket-connect host port → "sockN" (TCP client)
- socket-server ?host? port → "sockN" listening socket (SO_REUSEADDR, backlog 8)
- socket-accept server-chan → {:channel :host :port}

Sockets reuse the channel_table from Phase 5, so existing channel-read/
write/close/select all work on them. Host arg supports localhost,
0.0.0.0, IPv4 literal, or gethostbyname lookup.

Tcl `socket` command:
- socket host port → TCP client
- socket -server cb port → listening socket; auto-registers a fileevent
  on the server channel that fires `_sock-do-accept SRV CB` per readable
  event. _sock-do-accept (internal) accepts the pending client and calls
  the user's callback as `cb client-chan host port`.

puts channel detection now also recognizes "sockN" prefix (was only
"fileN") and dispatches to channel-write.

+4 idiom tests: socket-server-fires-callback, socket-client-server-
roundtrip, socket-server-peer-host, socket-multiple-connections.
358/358 green.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

hosts/ocaml/lib/sx_primitives.ml

@@ -3264,6 +3264,86 @@ let () =
       Nil
     | _ -> raise (Eval_error "channel-set-blocking!: (channel bool)"));
 
+  (* === Sockets === wrapping Unix.socket/connect/bind/listen/accept *)
+  let resolve_inet_addr host =
+    if host = "" || host = "0.0.0.0" then Unix.inet_addr_any
+    else if host = "localhost" then Unix.inet_addr_loopback
+    else
+      try Unix.inet_addr_of_string host
+      with _ ->
+        try
+          let entry = Unix.gethostbyname host in
+          if Array.length entry.Unix.h_addr_list = 0 then
+            raise (Eval_error ("socket: cannot resolve " ^ host))
+          else entry.Unix.h_addr_list.(0)
+        with Not_found -> raise (Eval_error ("socket: cannot resolve " ^ host))
+  in
+  let port_of v = match v with
+    | Integer n -> n
+    | Number n -> int_of_float n
+    | _ -> raise (Eval_error "socket: port must be a number")
+  in
+  let alloc_chan_name () =
+    let id = !channel_next_id in
+    incr channel_next_id;
+    Printf.sprintf "sock%d" id
+  in
+
+  register "socket-connect" (fun args ->
+    match args with
+    | [String host; port_v] ->
+      let port = port_of port_v in
+      let addr = Unix.ADDR_INET (resolve_inet_addr host, port) in
+      let sock = Unix.socket Unix.PF_INET Unix.SOCK_STREAM 0 in
+      (try Unix.connect sock addr
+       with Unix.Unix_error (e, _, _) ->
+         (try Unix.close sock with _ -> ());
+         raise (Eval_error ("socket-connect: " ^ Unix.error_message e)));
+      let name = alloc_chan_name () in
+      Hashtbl.replace channel_table name (sock, "rw", ref false, ref true);
+      String name
+    | _ -> raise (Eval_error "socket-connect: (host port)"));
+
+  register "socket-server" (fun args ->
+    let (host, port) = match args with
+      | [port_v] -> ("", port_of port_v)
+      | [String h; port_v] -> (h, port_of port_v)
+      | _ -> raise (Eval_error "socket-server: (port) or (host port)")
+    in
+    let addr = Unix.ADDR_INET (resolve_inet_addr host, port) in
+    let sock = Unix.socket Unix.PF_INET Unix.SOCK_STREAM 0 in
+    Unix.setsockopt sock Unix.SO_REUSEADDR true;
+    (try Unix.bind sock addr
+     with Unix.Unix_error (e, _, _) ->
+       (try Unix.close sock with _ -> ());
+       raise (Eval_error ("socket-server: bind: " ^ Unix.error_message e)));
+    Unix.listen sock 8;
+    let name = alloc_chan_name () in
+    Hashtbl.replace channel_table name (sock, "server", ref false, ref true);
+    String name);
+
+  register "socket-accept" (fun args ->
+    match args with
+    | [String name] ->
+      let (sock, _, _, _) = chan_get name in
+      let (client_sock, client_addr) =
+        try Unix.accept sock
+        with Unix.Unix_error (e, _, _) ->
+          raise (Eval_error ("socket-accept: " ^ Unix.error_message e))
+      in
+      let (host_str, port) = match client_addr with
+        | Unix.ADDR_INET (addr, p) -> (Unix.string_of_inet_addr addr, p)
+        | Unix.ADDR_UNIX path -> (path, 0)
+      in
+      let client_name = alloc_chan_name () in
+      Hashtbl.replace channel_table client_name (client_sock, "rw", ref false, ref true);
+      let d = Hashtbl.create 3 in
+      Hashtbl.replace d "channel" (String client_name);
+      Hashtbl.replace d "host" (String host_str);
+      Hashtbl.replace d "port" (Integer port);
+      Dict d
+    | _ -> raise (Eval_error "socket-accept: (server-channel)"));
+
   (* io-select-channels: (read-list write-list timeout-ms) → {:readable [...] :writable [...]}
      timeout-ms < 0 → block indefinitely; 0 → poll. Returns ready channel names. *)
   register "io-select-channels" (fun args ->