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/scoreboard.json

@@ -1,6 +1,6 @@
 {
-  "generated": "2026-05-05T12:16:51Z",
-  "total_pass": 508,
+  "generated": "2026-05-05T12:23:35Z",
+  "total_pass": 514,
   "total_fail": 0,
   "suites": [
     {"name": "Phase 1: tokenizer/reader", "pass": 79, "fail": 0},
@@ -14,6 +14,6 @@
     {"name": "Phase 4: geometry", "pass": 12, "fail": 0},
     {"name": "Phase 4: mop-trace", "pass": 13, "fail": 0},
     {"name": "Phase 5: macros+LOOP", "pass": 27, "fail": 0},
-    {"name": "Phase 6: stdlib", "pass": 44, "fail": 0}
+    {"name": "Phase 6: stdlib", "pass": 50, "fail": 0}
   ]
 }