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>

selfhost/backend/emit_new.ludic

@@ -26,6 +26,7 @@ function rec_field(rec: Node, fname: pointer) -> Node {
 function emit_new_struct(name: pointer, rec: Node) -> Val {
   let s = layout_node(name)                     # a struct or a property — same shape
   if (s == null) { perr("unknown record type in new") }
+  vis_check(s, name)
   let lty = layout_ty(name)
   let sz = emit_sizeof(lty)
   let obj = emit_bind(`call ptr @malloc(i64 {sz})`)