cl: Phase 6 FORMAT + substr fixes — 514/514 tests

FORMAT with ~A/~S/~D/~F/~%/~&/~T/~P/~{...~}/~^; cl-fmt-loop,
cl-fmt-find-close, cl-fmt-iterate, cl-fmt-a/cl-fmt-s helpers.
Fix substr(start,length) semantics throughout: SUBSEQ end formula
corrected to (- end start), cl-fmt-loop char extraction fixed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/common-lisp/tests/stdlib.sx

@@ -234,6 +234,32 @@
   (ev "(subseq \"hello\" 2)")
   "llo")
 
+;; ── FORMAT ─────────────────────────────────────────────────────────
+
+(check "format ~A"
+  (ev "(format nil \"hello ~A\" \"world\")")
+  "hello world")
+
+(check "format ~D"
+  (ev "(format nil \"~D items\" 42)")
+  "42 items")
+
+(check "format two args"
+  (ev "(format nil \"~A ~A\" 1 2)")
+  "1 2")
+
+(check "format ~A+~A=~A"
+  (ev "(format nil \"~A + ~A = ~A\" 1 2 3)")
+  "1 + 2 = 3")
+
+(check "format iterate"
+  (ev "(format nil \"~{~A~}\" (quote (1 2 3)))")
+  "123")
+
+(check "format iterate with space"
+  (ev "(format nil \"(~{~A ~})\" (quote (1 2 3)))")
+  "(1 2 3 )")
+
 ;; ── summary ──────────────────────────────────────────────────────
 
 (define stdlib-passed passed)