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_fnval.ludic

@@ -62,6 +62,7 @@ function fn_ty_ret(t: pointer) -> pointer {
 function emit_fnref(e: Node) -> Val {
   let d = find_fn(e.s)
   if d == null { perr(`fn {e.s}: no function called {e.s}`) }
+  vis_check(d, e.s)
   return val(`@fn_{e.s}`, fn_sig_of(d))
 }
 # a call through a value of a function type