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>

tools/docgen/inventory.json

@@ -25,7 +25,7 @@
   ],
   "builtins": [
     "fn-print",
-    "fn-str",
+    "fn-string",
     "fn-quit",
     "fn-save",
     "fn-load",
@@ -327,7 +327,7 @@
     "type-ptr",
     "type-ptrs",
     "type-slices",
-    "type-str",
+    "type-string",
     "type-void",
     "type-words"
   ],