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/modules/ports.ludic

@@ -0,0 +1,15 @@
+# ports.ludic — L3 ports: module clock needs the time and declares `port Clock`; module app binds
+# it with its own function; clock calls through `Clock.now()` and never names app. A member with a
+# default (`day`) may be left out of the bind.
+#
+# Running it prints:  30 5
+import "station/units"
+import "station/clock"
+import "station/app"
+program Ports {
+  entry {
+    let a = Clock.now()
+    advance(-25)
+    print(`{a} {hour_of_day()}`)
+  }
+}

examples/modules/station/app/index.ludic

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+# app/index.ludic - the app puts the program together: it sees the port and fills it
+module app uses clock
+bind Clock { now: fn game_hours }
+var g_hours: int = 30
+function game_hours() -> int {
+  return g_hours
+}
+export function advance(h: int) -> void {
+  g_hours += h
+}

examples/modules/station/clock/index.ludic

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+# clock/index.ludic - the clock needs the time of day and asks for it through a port: it names
+# no module that answers, so it `uses` only units
+module clock uses units
+export port Clock {
+  now: fn() -> int
+  day: fn() -> int = fn first_day
+}
+function first_day() -> int {
+  return 1
+}
+export function hour_of_day() -> int {
+  return Clock.now() - (Clock.day() - 1) * HOURS
+}

examples/modules/station/units/index.ludic

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+# units/index.ludic - a module with nothing to ask of anyone
+module units
+export const HOURS: int = 24