Link AsciiDoc cross references to their source

`xref:other.adoc[]` linked to `other.html`, which doesn't exist in the
repository, so following it showed a missing file instead.

tests/unit/asciidoc.test.ts

@@ -0,0 +1,21 @@
+import { expect, test, vi } from "vitest";
+
+import { renderAsciidoc } from "@app/lib/asciidoc";
+
+vi.mock("@app/lib/invoke", () => ({ invoke: vi.fn() }));
+
+const source = {
+  rid: "rad:z3fpY7nttPPa6MBnAv2DccHzQJnqe",
+  sha: "a".repeat(40),
+  path: "docs/index.adoc",
+};
+
+test.each([
+  ["xref:spec.adoc[Spec]", "spec.adoc"],
+  ["xref:spec.adoc#_details[Spec]", "spec.adoc#_details"],
+  ["<<spec.adoc#,Spec>>", "spec.adoc"],
+  ["<<_details,Details>>", "#_details"],
+])("links %j to %j", async (content, href) => {
+  const html = await renderAsciidoc(content, source);
+  expect(html).toContain(`href="${href}"`);
+});