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>

examples/rejected/port_unbound.ludic

@@ -0,0 +1,7 @@
+# L3: a port that is used and never bound is a program not yet put together
+program PortUnbound {
+  port Clock { now: fn() -> int }
+  entry {
+    print(Clock.now())
+  }
+}