feat(core): adopt remark/unified as the internal Markdown parser (ADR 0002)

Add unified + remark-parse as dependencies of @kitchen-md/core and wire a
private internal processor (packages/core/src/markdown.ts) that parses
CommonMark into mdast. remark stays an implementation detail and is not
re-exported from the package entry, so consumers take no transitive remark
dependency (ADR 0003).

The OFM wikilink plugin and the public parse/AST surface are deferred to the
core-parser spec: canonical remark-wiki-link@2.0.1 targets an old micromark
generation incompatible with remark-parse@11, and OFM plus annotation
handling is that spec's scope.

This also exercises the flake's bun2nix vendoring against a real dependency
closure (58 vendored packages); nix build still compiles a working binary
offline in the sandbox.

packages/core/package.json

@@ -8,5 +8,12 @@
   },
   "scripts": {
     "test": "bun test"
+  },
+  "dependencies": {
+    "remark-parse": "^11.0.0",
+    "unified": "^11.0.5"
+  },
+  "devDependencies": {
+    "@types/mdast": "^4.0.4"
   }
 }

packages/core/src/markdown.ts

@@ -0,0 +1,10 @@
+// core's internal CommonMark parser, built on remark/unified.
+import type { Root } from "mdast";
+import remarkParse from "remark-parse";
+import { unified } from "unified";
+
+const processor = unified().use(remarkParse);
+
+export function parseMarkdown(source: string): Root {
+  return processor.parse(source);
+}

packages/core/src/markdown_test.ts

@@ -0,0 +1,20 @@
+import { describe, expect, test } from "bun:test";
+import { parseMarkdown } from "./markdown.ts";
+
+describe("internal markdown parser", () => {
+  test("parses CommonMark into an mdast root", () => {
+    const tree = parseMarkdown("# Title\n\nA paragraph.\n");
+
+    expect(tree.type).toBe("root");
+    expect(tree.children[0]?.type).toBe("heading");
+    expect(tree.children[1]?.type).toBe("paragraph");
+  });
+
+  test("represents a fenced code block as a code node with its value verbatim", () => {
+    const tree = parseMarkdown("```\n@ladle{}\n```\n");
+
+    const first = tree.children[0];
+    expect(first?.type).toBe("code");
+    expect(first).toMatchObject({ value: "@ladle{}" });
+  });
+});