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/modules/reg/alpha/index.ludic

@@ -0,0 +1,6 @@
+# alpha/index.ludic - another module's entry
+module alpha uses core
+def Systems animals { run: fn animals_run }
+function animals_run() -> int {
+  return 2
+}