feat(lang): ports - port Clock { ... } in a module, bind Clock { ... } in the app

A port is a record of function values a module calls through
(Clock.now()) without naming the module that answers. A port used and
never bound, a bind missing a required member or naming one the port
lacks, and a second bind are refused; the binder must see the port, and
what it binds is checked from its own file. Reseed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

selfhost/backend/emit_head.ludic

@@ -72,7 +72,14 @@ function emit_global_init_fn() -> void {
       # may see (L3), are that file's - not whichever statement was lowered last
       g_err_file = d.file
       g_err_line = d.line
+      # a port's bind is written in the app and checked there (frontend/ports.ludic)
+      if is_port_var(d) {
+        g_err_file = d.a.file
+        g_err_line = d.a.line
+        g_vis_off = true
+      }
       let v = emit_expr(d.a)
+      g_vis_off = false
       let cv = coerce_code(v, d.ty)                 # any conversion is its own line, before the store
       emit("  store ");  emit(llty(d.ty));  emit(" ");  emit(cv);  emit(", ptr @g_");  emit(d.s);  emit("\n")
     }

selfhost/backend/emit_vis.ludic

@@ -30,6 +30,7 @@ function vis_say(m: pointer) -> void {
 }
 function vis_check(d: Node, what0: pointer) -> void {
   if d == null { return }
+  port_check_bound(d)
   if g_vis_off { return }
   if d.file == null { return }
   let what = vis_plain(what0)