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.md

@@ -1,6 +1,6 @@
 # Common Lisp on SX — Scoreboard
 
-_Generated: 2026-05-05 12:16 UTC_
+_Generated: 2026-05-05 12:23 UTC_
 
 | Suite | Pass | Fail | Status |
 |-------|------|------|--------|
@@ -15,6 +15,6 @@ _Generated: 2026-05-05 12:16 UTC_
 | Phase 4: geometry | 12 | 0 | pass |
 | Phase 4: mop-trace | 13 | 0 | pass |
 | Phase 5: macros+LOOP | 27 | 0 | pass |
-| Phase 6: stdlib | 44 | 0 | pass |
+| Phase 6: stdlib | 50 | 0 | pass |
 
-**Total: 508 passed, 0 failed**
+**Total: 514 passed, 0 failed**