feat(lang): L3 module scope - module, export, friend module

A barrel's 'module NAME' makes its directory a module; a declaration other modules
use says 'export'. Private use from another module is an error naming the module
and where to mark it; 'friend module' sees everything (a test harness); a file in
no module is public and a package keeps its own module. LUDIC_VIS_REPORT=1 lists
every violation instead of stopping, so a codebase can be given its exports first.

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

examples/modules/bank/index.ludic

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+# bank/index.ludic - a module: only what it exports is reachable from outside
+module bank
+import "ledger.ludic"

examples/modules/bank/ledger.ludic

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+# bank/ledger.ludic - inherits `module bank` from the barrel that imports it
+var balance: int = 0
+export event Deposited { amount: int }
+function add(n: int) -> void {
+  balance += n
+}
+export function deposit(n: int) -> void {
+  add(n)
+  emit Deposited(amount: n)
+}
+export function total() -> int {
+  return balance
+}

examples/modules/visible.ludic

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+# visible.ludic — L3: a program in no module uses what module bank exports: a function to call
+# and an event to listen to. Its private `add` and `balance` stay out of reach.
+#
+# Running it prints:  42 42
+import "bank"
+program Visible {
+  var heard: int = 0
+  @On(Deposited) handler Heard { heard += amount }
+  entry {
+    deposit(30)
+    deposit(12)
+    print(`{total()} {heard}`)
+  }
+}