test: move parser tests to parse_test.ts, off the barrel (task 0003)

packages/core/src/index.ts is a pure re-export barrel with no behavior
of its own, so its behavior tests belonged next to the module they
exercise. Relocate the parser tests (and their roadmap todos) into a
co-located parse_test.ts importing parse from ./parse.ts, and drop
index_test.ts. A broken re-export is still caught by typecheck and by
the bin package importing parse through the public entrypoint.

packages/core/src/parse_test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, test } from "bun:test";
 
-import { parse } from "@kitchen-md/core";
+import { parse } from "./parse.ts";
 
 describe("parser", () => {
   describe("frontmatter", () => {