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>

selfhost/tests/long.ludic

@@ -18,6 +18,6 @@ program T {
     print(x == x)                 # 1
     print(0 - sq)                 # -1000000000000 (unary negate stays 64-bit)
     print(big)                    # 1000000
-    print(Text.length(str(sq)))   # 13 — str(long) renders all 13 digits
+    print(Text.length(string(sq)))   # 13 — string(long) renders all 13 digits
   }
 }