cl: Phase 6 stdlib — sequence/list/string functions, 508/508 tests

mapc/mapcan/reduce/find/find-if/position/count/every/some/notany/
notevery/remove/remove-if/subst/member; assoc/rassoc/getf/last/
butlast/nthcdr/list*/cadr/caddr/cadddr; subseq/coerce/make-list.
44 new tests in tests/stdlib.sx. Helpers: cl-member-helper,
cl-subst-helper, cl-position-helper.

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 12:00 UTC_
+_Generated: 2026-05-05 12:16 UTC_
 
 | Suite | Pass | Fail | Status |
 |-------|------|------|--------|
@@ -15,5 +15,6 @@ _Generated: 2026-05-05 12:00 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 |
 
-**Total: 464 passed, 0 failed**
+**Total: 508 passed, 0 failed**