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/open_registry.ludic

@@ -0,0 +1,17 @@
+# open_registry.ludic — L8 + L3: `export open registry Systems of System` takes defs from other
+# modules. The index order is the declaring module's entries, then every other module's by module
+# name - alpha before zeta, although zeta is imported first - so it never depends on import order.
+#
+# Running it prints:  clock animals weather / 2 2 3
+import "reg/core"
+import "reg/zeta"
+import "reg/alpha"
+program OpenRegistry {
+  entry {
+    var out = ""
+    for i in 0 .. SY_COUNT { out = out + Systems[i].key + " " }
+    let s = Systems[SY_ANIMALS]
+    let w = Systems[SY_WEATHER]
+    print(`{out}/ {s.run()} {SY_WEATHER} {w.run()}`)
+  }
+}

examples/modules/reg/alpha/index.ludic

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+# 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

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+# 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

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+# 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
+}