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>

plans/common-lisp-on-sx.md

@@ -107,7 +107,7 @@ Core mapping:
 - [x] Sequence functions — `mapcar`, `mapc`, `mapcan`, `reduce`, `find`, `find-if`, `position`, `count`, `every`, `some`, `notany`, `notevery`, `remove`, `remove-if`, `subst`
 - [x] List ops — `assoc`, `getf`, `nth`, `last`, `butlast`, `nthcdr`, `tailp`, `ldiff`
 - [x] String ops — `string=`, `string-upcase`, `string-downcase`, `subseq`, `concatenate`
-- [ ] FORMAT — basic directives `~A`, `~S`, `~D`, `~F`, `~%`, `~&`, `~T`, `~{...~}` (iteration), `~[...~]` (conditional), `~^` (escape), `~P` (plural)
+- [x] FORMAT — basic directives `~A`, `~S`, `~D`, `~F`, `~%`, `~&`, `~T`, `~{...~}` (iteration), `~[...~]` (conditional), `~^` (escape), `~P` (plural)
 - [ ] Drive corpus to 200+ green
 
 ## SX primitive baseline
@@ -124,6 +124,8 @@ data; format for string templating.
 
 _Newest first._
 
+- 2026-05-05: Phase 6 FORMAT — cl-fmt-a/cl-fmt-s/cl-fmt-find-close/cl-fmt-iterate/cl-fmt-loop in eval.sx; ~A/~S/~D/~F/~%/~&/~T/~P/~{...~}/~[...~]/~^/~~; also fixed substr(start,length) semantics throughout (SUBSEQ, cl-fmt-loop); 6 FORMAT tests added to stdlib.sx; 514 total tests, 0 failed.
+
 - 2026-05-05: Phase 6 stdlib — sequence functions (mapc/mapcan/reduce/find/find-if/find-if-not/position/position-if/count/count-if/every/some/notany/notevery/remove/remove-if/remove-if-not/subst/member), list ops (assoc/rassoc/getf/last/butlast/nthcdr/copy-list/list*/caar/cadr/cdar/cddr/caddr/cadddr/pairlis), string ops (subseq/string/char/string-length/string</>), plus coerce/make-list/write-to-string; 44 tests in tests/stdlib.sx; Phase 6 sequence+list+string boxes ticked. Total: 508 tests, 0 failed.
 
 - 2026-05-05: Phase 4 CLOS fully complete — `lib/common-lisp/clos.sx` (27 forms): clos-class-registry (8 built-in classes), defclass/make-instance/slot-value/slot-boundp/set-slot-value!/find-class/change-class, defgeneric/defmethod with :before/:after/:around, clos-call-generic (standard method combination: sort by specificity, fire befores, call primary chain, fire afters in reverse), call-next-method/next-method-p, with-slots, accessor installation; 41 tests in `tests/clos.sx`; classic programs `geometry.sx` (12 tests, multi-dispatch intersect on P/L/Plane) and `mop-trace.sx` (13 tests, :before/:after tracing). Dynamic variables in eval.sx: cl-apply-dyn saves/restores global bindings around let for specials (cl-mark-special!/cl-special?/cl-dyn-unbound). Key gotchas: qualifier strings are "before"/"after"/"around" (no colon); dict-set pure = assoc; dict->list = (map (fn (k) (list k (get d k))) (keys d)); clos-add-reader-method bootstrapped via set! after defmethod defined; test isolation: use unique var names to avoid *y* collision. 437 total tests, 0 failed.