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/hidden_kit/index.ludic

@@ -0,0 +1,10 @@
+# hidden_kit/index.ludic - open, but not exported: nothing outside kit can name it
+module kit
+export property Tool {
+  name: string = ""
+}
+open registry Tools of Tool
+def Tools axe { name: "Axe" }
+export function tool_count() -> int {
+  return TOOLS_COUNT
+}