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/x/prelude.ludic

@@ -113,7 +113,7 @@ function check(label: ptr, got: ptr, want: ptr) -> void {
 # print the "== N passed, M failed ==" footer and return the process exit code
 function report() -> int {
   print("")
-  print(`== {str(PASS)} passed, {str(FAIL)} failed ==`)
+  print(`== {string(PASS)} passed, {string(FAIL)} failed ==`)
   if (FAIL == 0) { return 0 }
   return 1
 }