ci: weekly store mutation run and replay fuzzing

`just mutants-store` (64 random cases per mutant, temp books on a RAM
disk in CI) runs beside the core's weekly; fuzz_store_replay joins the
weekly fuzz matrix. docs/testing.md lists the spec-blind store suite,
the refusal property's converse law, and the replay fuzz oracles.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GWVo7x3UHw5kwCUaWxiejX

.gitignore

@@ -3,3 +3,5 @@
 .DS_Store
 site/dist/
 .netlify/
+/mutants.out*/
+/mutants-store/