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/conformance.sh

@@ -107,6 +107,10 @@ run_suite "Phase 5: macros+LOOP" \
   "lib/common-lisp/reader.sx lib/common-lisp/parser.sx lib/common-lisp/eval.sx lib/common-lisp/loop.sx lib/common-lisp/tests/macros.sx" \
   "macro-passed" "macro-failed" "macro-failures"
 
+run_suite "Phase 6: stdlib" \
+  "lib/common-lisp/reader.sx lib/common-lisp/parser.sx lib/common-lisp/eval.sx lib/common-lisp/tests/stdlib.sx" \
+  "stdlib-passed" "stdlib-failed" "stdlib-failures"
+
 echo ""
 echo "=== Total: $TOTAL_PASS passed, $TOTAL_FAIL failed ==="