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_clock/index.ludic

@@ -0,0 +1,6 @@
+# port_clock/index.ludic - a port that is not exported
+module clock
+port Clock { now: fn() -> int }
+export function tell() -> int {
+  return Clock.now()
+}

examples/rejected/port_missing.ludic

@@ -0,0 +1,14 @@
+# L3: a bind gives every member that has no default
+program PortMissing {
+  port Clock {
+    now: fn() -> int
+    day: fn() -> int
+  }
+  function hours() -> int {
+    return 3
+  }
+  bind Clock { now: fn hours }
+  entry {
+    print(Clock.now())
+  }
+}

examples/rejected/port_private.ludic

@@ -0,0 +1,11 @@
+# L3: whoever binds a port has to see it - clock's Clock is not exported
+import "port_clock"
+program PortPrivate {
+  function hours() -> int {
+    return 3
+  }
+  bind Clock { now: fn hours }
+  entry {
+    print(tell())
+  }
+}

examples/rejected/port_twice.ludic

@@ -0,0 +1,15 @@
+# L3: a port is bound once
+program PortTwice {
+  port Clock { now: fn() -> int }
+  function hours() -> int {
+    return 3
+  }
+  function minutes() -> int {
+    return 4
+  }
+  bind Clock { now: fn hours }
+  bind Clock { now: fn minutes }
+  entry {
+    print(Clock.now())
+  }
+}

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())
+  }
+}