refactor(epub): replace Kroki HTTP dependency with local mmdc rendering (#10) (#52)

* 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

.github/workflows/release.yml

@@ -17,6 +17,14 @@ jobs:
       - name: Install uv
         uses: astral-sh/setup-uv@v4
 
+      - name: Install Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+
+      - name: Install Mermaid CLI
+        run: npm install -g @mermaid-js/mermaid-cli
+
       - name: Build EPUB
         run: uv run scripts/build_epub.py