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/frontend/ast.ludic

@@ -85,6 +85,7 @@ property Node {
   kids: []Node                # variadic children
   line: int = 0
   file: pointer = null        # the source file the node was parsed from (for diagnostics)
+  vis:  int = 0               # L3: 1 when the declaration is `export`ed from its module
 }
 
 # every node remembers where it was parsed (file + the line of the token the