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.json

@@ -1,6 +1,6 @@
 {
-  "generated": "2026-05-05T12:00:17Z",
-  "total_pass": 464,
+  "generated": "2026-05-05T12:16:51Z",
+  "total_pass": 508,
   "total_fail": 0,
   "suites": [
     {"name": "Phase 1: tokenizer/reader", "pass": 79, "fail": 0},
@@ -13,6 +13,7 @@
     {"name": "Phase 4: CLOS", "pass": 41, "fail": 0},
     {"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 5: macros+LOOP", "pass": 27, "fail": 0},
+    {"name": "Phase 6: stdlib", "pass": 44, "fail": 0}
   ]
 }