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/builtins/fn-arg.md

@@ -4,7 +4,7 @@ name: arg
 category: builtins
 kind: builtin
 tokens: arg
-sig: arg(i) -> str
+sig: arg(i) -> string
 tip: The i-th command-line argument as a string.
 order: 50
 ---

docs/language/builtins/fn-getenv.md

@@ -4,7 +4,7 @@ name: getenv
 category: builtins
 kind: builtin
 tokens: getenv
-sig: getenv(name) -> str
+sig: getenv(name) -> string
 tip: Read an environment variable, returning empty when it is unset.
 order: 23
 ---

docs/language/builtins/fn-string.md

@@ -1,15 +1,15 @@
 ---
-id: fn-str
-name: str
+id: fn-string
+name: string
 category: builtins
 kind: builtin
-tokens: str
-sig: str(x) -> str
+tokens: string
+sig: string(x) -> string
 tip: Convert an int, bool, or fixed value to text; a string passes through unchanged.
 order: 1
 ---
 
-Converts a value to its textual form: an <code>int</code>, <code>bool</code>, or <code>fixed</code> becomes a string, and a value that is already a string is returned unchanged. This is what backtick <code>`{…}`</code> interpolation calls under the hood, so most of the time you can interpolate directly instead of calling <code>str</code> yourself. Reach for the explicit form when you need to store or pass around the text, or build a string in pieces. The result can be printed, drawn with <code>Screen.draw_text</code>, or concatenated.
+Converts a value to its textual form: an <code>int</code>, <code>bool</code>, or <code>fixed</code> becomes a string, and a value that is already a string is returned unchanged. This is what backtick <code>`{…}`</code> interpolation calls under the hood, so most of the time you can interpolate directly instead of calling <code>string</code> yourself. Reach for the explicit form when you need to store or pass around the text, or build a string in pieces. The result can be printed, drawn with <code>Screen.draw_text</code>, or concatenated.
 
 Parameters:
 - `x` — the value to convert to text
@@ -19,7 +19,7 @@ program LabelValue {
   var score: int = 250
 
   handler ReportScore phase Start {
-    let label = str(score)
+    let label = string(score)
     print(label)
   }
 }