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
+}

examples/modules/reg/core/index.ludic

@@ -0,0 +1,11 @@
+# core/index.ludic - declares the registry other modules add to, and its own first entry
+module core
+export property System {
+  key: string = ""
+  run: fn() -> int = null
+}
+export open registry Systems of System as SY
+def Systems clock { run: fn clock_run }
+function clock_run() -> int {
+  return 1
+}

examples/modules/reg/zeta/index.ludic

@@ -0,0 +1,6 @@
+# zeta/index.ludic - adds an entry to core's open registry; its function stays private to zeta
+module zeta uses core
+def Systems weather { run: fn weather_run }
+function weather_run() -> int {
+  return 3
+}