test: refactor csv-loader tests into separate files

Split the monolithic `src/csv-loader/loader.test.ts` into multiple
specialized test files to improve maintainability and readability:

- `parseCsv-basic.test.ts`: Primitive types, arrays, and tuples
- `parseCsv-caching.test.ts`: Table caching logic
- `parseCsv-circular.test.ts`: Circular reference detection
- `parseCsv-combinators.test.ts`: References in unions and tuples
- `parseCsv-noResolveRefs.

src/csv-loader/tests/parseCsv-circular.test.ts

@@ -0,0 +1,53 @@
+import { describe, it, expect } from "vitest";
+import { parseCsv } from "../loader";
+import * as path from "path";
+import { fixturesDir, readFixture } from "../test-utils";
+
+describe("parseCsv - circular reference detection", () => {
+  it("should detect self-referencing circular reference", () => {
+    const csv = ["id,name,parent", "string,string,@self_ref", "1,Root,2"].join(
+      "\n",
+    );
+
+    expect(() =>
+      parseCsv(csv, {
+        emitTypes: false,
+        currentFilePath: path.join(fixturesDir, "self_ref.csv"),
+      }),
+    ).toThrow(/Circular reference detected/);
+  });
+
+  it("should detect mutual circular reference (A -> B -> A)", () => {
+    const csv = readFixture("circular_a.csv");
+
+    expect(() =>
+      parseCsv(csv, {
+        emitTypes: false,
+        currentFilePath: path.join(fixturesDir, "circular_a.csv"),
+      }),
+    ).toThrow(/Circular reference detected/);
+  });
+
+  it("should allow same table referenced from multiple columns without circular reference", () => {
+    const usersCsv = readFixture("users.csv");
+    const csv = ["id,creator,reviewer", "string,@users,@users", "1,1,2"].join(
+      "\n",
+    );
+
+    const result = parseCsv(csv, {
+      emitTypes: false,
+      currentFilePath: path.join(fixturesDir, "test.csv"),
+    });
+
+    expect(result.data[0].creator).toEqual({
+      id: "1",
+      name: "Alice",
+      email: "alice@example.com",
+    });
+    expect(result.data[0].reviewer).toEqual({
+      id: "2",
+      name: "Bob",
+      email: "bob@example.com",
+    });
+  });
+});