Extract engine from citation-evidence umbrella (CENG-WP-0001)

Bootstrap @citation-evidence/engine as a standalone TypeScript package
with shared types and engine services copied from the umbrella MVP.
All 89 tests pass with lint and typecheck clean.

src/engine/rendering/index.ts

@@ -0,0 +1,8 @@
+export {
+  renderCitationCardMarkdown,
+  type RenderCitationCardMarkdownInput,
+} from "./markdown";
+export {
+  renderCitationCardHtml,
+  type RenderCitationCardHtmlInput,
+} from "./html";