ludic.ui, runtime, compiler: an expression's Value comes from the screen's pool while it is built (never a state's start or an action's), true and false shared, calc() terms pooled and read in place, a model's list fields filled in place (value_set_ints/strs/floats/bools); reseeded

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

runtime/native/namespaces.ludic

@@ -480,6 +480,10 @@ namespace Value {
   alias as_str(value) = value_as_str
   alias free(value) = value_free
   alias clear(value) = value_clear
+  alias become(value, tag) = value_become
+  alias be_int(value, n) = value_be_int
+  alias be_float(value, x) = value_be_float
+  alias be_str(value, s) = value_be_str
 }
 namespace Json {
   alias encode(value) = json_encode