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>

lib/tcl/tests/idioms.sx

@@ -274,6 +274,35 @@
         :result)
       "1")
 
+    ; 38-41. Phase 5c sockets: TCP client + server
+    (ok "socket-server-fires-callback"
+      (get
+        (run
+          "proc h {sock host port} { global got; set got hit; close $sock }\nset srv [socket -server h 18901]\nset cli [socket localhost 18901]\nvwait got\nclose $srv\nclose $cli\nset got")
+        :result)
+      "hit")
+
+    (ok "socket-client-server-roundtrip"
+      (get
+        (run
+          "proc h {sock host port} { global received; gets $sock line; set received $line; close $sock }\nset srv [socket -server h 18902]\nset cli [socket localhost 18902]\nputs $cli ping\nflush $cli\nvwait received\nclose $srv\nclose $cli\nset received")
+        :result)
+      "ping")
+
+    (ok "socket-server-peer-host"
+      (get
+        (run
+          "proc h {sock host port} { global peer; set peer $host; close $sock }\nset srv [socket -server h 18903]\nset cli [socket 127.0.0.1 18903]\nvwait peer\nclose $srv\nclose $cli\nset peer")
+        :result)
+      "127.0.0.1")
+
+    (ok "socket-multiple-connections"
+      (get
+        (run
+          "proc h {sock host port} { global count; incr count; close $sock }\nset count 0\nset srv [socket -server h 18904]\nset c1 [socket localhost 18904]\nset c2 [socket localhost 18904]\nset c3 [socket localhost 18904]\nwhile {$count < 3} { update; after 5 }\nclose $srv\nclose $c1\nclose $c2\nclose $c3\nset count")
+        :result)
+      "3")
+
     (dict
       "passed"
       tcl-idiom-pass