Implement CE-WP-0004 T01-T05: citation card export (Markdown + HTML)

Per-evidence-item export: click Export → Copy as Markdown / Copy as HTML
writes a portable citation card to the clipboard. Cmd/Ctrl+Shift+C
exports the active evidence as Markdown.

- ADR-0007 locks the Markdown + HTML output formats.
- New shared types: CitationCard, openContextUrl(), resolveSourceLabel().
- Engine renderers under src/engine/rendering/: renderCitationCardMarkdown,
  renderCitationCardHtml — snapshot-tested, escape-safe, BEM classes for HTML.
- src/work/useExportEvidence.ts wires engine + renderers + clipboard.
- EvidenceSidebar gains an Export popover per row + auto-dismissing toast.
- E2E test (tests/integration/citation-card-export-e2e.dom.test.tsx)
  walks PRD scenario steps 10-11 and asserts the clipboard payload.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/engine/index.ts

@@ -27,6 +27,7 @@ import {
 export * from "./events";
 export * from "./repos";
 export * from "./services";
+export * from "./rendering";
 export {
   SNAPSHOT_VERSION,
   attachPersister,