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/shared/open-context-url.ts

@@ -0,0 +1,36 @@
+/**
+ * Open-context URL — the canonical deep link that lets a citation card
+ * reopen the source context that backs it.
+ *
+ * Implements `wiki/ArchitectureOverview.md` §14.3. The URL shape is:
+ *
+ *     /viewer?document=<documentId>&annotation=<annotationId>
+ *
+ * Both ids are mandatory. The annotation alone is insufficient because
+ * the router needs the document id to mount the right viewer adapter
+ * before resolving the annotation's selectors.
+ *
+ * This convention is stable across persistence modes — when the
+ * in-memory engine is replaced by a real backend (ADR-0005), the same
+ * URL shape is expected to still resolve.
+ */
+
+import type { AnnotationId, DocumentId } from "./ids";
+
+export interface OpenContextUrlInput {
+  readonly documentId: DocumentId;
+  readonly annotationId: AnnotationId;
+}
+
+/**
+ * Build the deep link for a given (document, annotation) pair.
+ *
+ * Query-string values are percent-encoded via `encodeURIComponent` so that
+ * any future id scheme containing reserved characters (`&`, `=`, `#`) still
+ * round-trips.
+ */
+export function openContextUrl(input: OpenContextUrlInput): string {
+  const docPart = encodeURIComponent(input.documentId);
+  const annPart = encodeURIComponent(input.annotationId);
+  return `/viewer?document=${docPart}&annotation=${annPart}`;
+}