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/rejected/private_module.ludic

@@ -0,0 +1,8 @@
+# L3: `add` is private to module bank; calling it from outside the module is refused
+import "private_bank"
+program Private {
+  entry {
+    add(5)
+    print(total())
+  }
+}