feat(lang): a module's private names are its own

A private function, var or const no longer collides with the same
spelling in another module, in no module or in the runtime: where two
meet, each private one is renamed for its module (seed$shop), with every
reference its module writes that no local shadows. Exported names stay
one namespace. Nothing is renamed without a clash. Reseed.

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

examples/rejected/exported_twins.ludic

@@ -0,0 +1,8 @@
+# L3: two modules that both EXPORT a name clash - only private names are a module's own
+import "exported_twins/a"
+import "exported_twins/b"
+program ExportedTwins {
+  entry {
+    print(seed())
+  }
+}

examples/rejected/exported_twins/a/index.ludic

@@ -0,0 +1,5 @@
+# a/index.ludic - exports seed
+module a
+export function seed() -> int {
+  return 1
+}

examples/rejected/exported_twins/b/index.ludic

@@ -0,0 +1,5 @@
+# b/index.ludic - exports seed too: exported names are one namespace
+module b
+export function seed() -> int {
+  return 2
+}