refactor(lang): rename the str type (and stringify builtin) to string

Expand the abbreviated string type and its conversion builtin to the full
word everywhere:
  str            ->  string        (the immutable-string type)
  str(x) -> str  ->  string(x) -> string   (the stringify builtin;
                                            what `{…}` interpolation calls)

Types are recognized by identifier, and llty maps both spellings to LLVM
`ptr`, so this is an atomic source rewrite: type annotations, the Ludic
type tags, the builtin name/dispatch, and the interpolation desugar, plus
the grammars, LSP, docs (type-str -> type-string, fn-str -> fn-string), and
inventory. int/bool stay (universally accepted, like Math).

Reseeded; C-free fixpoint holds; all suites green (45/24/29); site + check.py OK.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/language/ecs/fn-world_get.md

@@ -30,7 +30,7 @@ program ReadField {
     let column_field = world_field_id(position_property, "column")
     for (position) in query [Position, {Player}] {
       let value = world_get(self(), position_property, column_field)
-      Screen.status(str(value))
+      Screen.status(string(value))
     }
   }
 }