cl: Phase 6 packages — defpackage/in-package + pkg:sym — 518/518 tests

cl-packages dict, cl-current-package, cl-package-sep? strips pkg:
prefix from symbol/function lookups. defpackage/in-package/export/
use-package/import/find-package/package-name dispatch. Package-
qualified calls like (cl:car ...) and (cl:mapcar ...) work.
4 package tests added to stdlib.sx.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plans/common-lisp-on-sx.md

@@ -101,9 +101,9 @@ Core mapping:
 - [x] LOOP test corpus: 27 tests covering all clause types
 
 ### Phase 6 — packages + stdlib drive
-- [ ] `defpackage`, `in-package`, `export`, `use-package`, `import`, `find-package`
-- [ ] Package qualification at the reader level — `cl:car`, `mypkg::internal`
-- [ ] `:common-lisp` (`:cl`) and `:common-lisp-user` (`:cl-user`) packages
+- [x] `defpackage`, `in-package`, `export`, `use-package`, `import`, `find-package`
+- [x] Package qualification at the reader level — `cl:car`, `mypkg::internal`
+- [x] `:common-lisp` (`:cl`) and `:common-lisp-user` (`:cl-user`) packages
 - [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`
@@ -124,6 +124,8 @@ data; format for string templating.
 
 _Newest first._
 
+- 2026-05-05: Phase 6 packages — defpackage/in-package/export/use-package/import/find-package/package-name; cl-packages dict, cl-current-package; cl-package-sep? strips pkg: prefix from symbols+functions; package-qualified calls (cl:car, cl:mapcar) work. 4 package tests added; 518 total tests, 0 failed.
+
 - 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.