v2.2.5 — ultrareview fixes

Bundle of seven correctness fixes surfaced by an independent ultrareview of 2.2.4. Semver-patch — no public API changes. Behavior changes are limited to outputs that were previously buggy: collision overwrites, mangled fenced code, silent broken images, dropped Tabs blocks, malformed-hash crashes, and the auto-closing dropdown.

Fixed

• intro.md collision on slug: '/' routes — sites with both docs/intro.md and a doc routed to /docs/ had their build output collide on intro.md, with the second route silently overwriting the first. Trailing-slash routes now resolve to index.md.
• Code fences corrupted by MDX/Docusaurus transforms — docs demonstrating Docusaurus syntax inside fenced code blocks had imports stripped and <Tabs> rewritten right out of their own examples. Added a fence-aware protection pass that handles backtick and tilde fences (length ≥ 3, ≤3 leading spaces, supports CRLF and unclosed fences), and supports legitimate <Tabs> blocks containing fenced code.
• Image dirs only copied to the first route's destination — sibling docs in the same source dir routed to different URL spaces had silent broken images. Switched to Map<src, Set<dest>> so each source dir is copied to every destination it serves.
• <TabItem> and YouTube iframe regexes rejected valid attribute orders — reverse-order TabItems silently emptied the entire <Tabs> block; reverse-order YouTube iframes survived as raw HTML. Both now parse attrs independently with single- or double-quote support.
• Multi-line and side-effect imports now stripped (.*? couldn't cross newlines, import './x.css'; wasn't matched).
• Component scrubber now backreferences the opening tag so siblings like <Outer><Inner>x</Inner></Outer> no longer leave an orphan </Outer>. (Deeply nested same-name components remain a known regex limitation.)
• <details> allows attributes on the opening tag and mixed-content summaries; body cleanup only trims outer blank padding so 4-space indents and intentional blank lines survive.
• Root.js decodeURIComponent crash — malformed hashes like #%foo no longer crash the scroll-to-anchor effect.
• Root.js scroll-to-anchor timer/listener leaks — quickly clicking different hash links no longer piles up timers that could scroll the page out from under the user.
• Dropdown copy-reset timer no longer auto-closes the menu or leaks on unmount.

Internal

• Added a node --test suite (zero new dependencies). Run with npm test.
• Extracted cleanMarkdownForDisplay, fence protection, image mapping, URL building, and hash decoding into focused lib/ modules for testability.

PR: #9