* test(scripts): cover repo-root boundary in check_cross_references
Locks in the boundary behavior added in #91: links resolving outside
the repo root are silently skipped, in-repo broken links still error,
and valid in-repo links still pass.
* test(scripts): cover missing-README detection in numbered lesson dirs
* refactor(epub): replace Kroki HTTP dependency with local mmdc rendering (#10)
Remove httpx/tenacity dependencies and Kroki.io API calls from the EPUB
build pipeline. MermaidRenderer now invokes mmdc (mermaid-cli) as a local
subprocess, eliminating intermittent CI failures caused by network
unavailability. CI workflow installs @mermaid-js/mermaid-cli before build.
* fix(epub): add subprocess timeout and fix deduplication log count
- Add 60s timeout to mmdc subprocess.run to prevent hanging builds
when Chromium/Puppeteer stalls in headless CI environments
- Fix misleading log that printed unique/total as equal counts;
now logs "N unique diagrams (M total blocks)" explicitly
- Add test_render_all_timeout and strengthen deduplication assertion