feat(lang): open registry - other modules' defs, in an order imports cannot change

A def from another module into a registry that is not open is refused,
and defs go through visibility (the registry exported, its module in the
definer's uses). Index order: the declaring module's entries, then the
other modules' by module name, each in reading order. A registry entry
is emitted as its own file's code, so what it names is seen from its
own module. Reseed.

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

examples/rejected/registry_hidden.ludic

@@ -0,0 +1,8 @@
+# L8 + L3: a def goes through visibility like any reference - the registry must be exported
+import "hidden_kit"
+program RegistryHidden {
+  def Tools saw { name: "Saw" }
+  entry {
+    print(tool_count())
+  }
+}