test(scripts): cover repo-root boundary in check_cross_references (#95)

* test(scripts): cover repo-root boundary in check_cross_references

Locks in the boundary behavior added in #91: links resolving outside
the repo root are silently skipped, in-repo broken links still error,
and valid in-repo links still pass.

* test(scripts): cover missing-README detection in numbered lesson dirs

scripts/tests/test_check_cross_references.py

@@ -0,0 +1,61 @@
+"""Tests for check_cross_references.py — focus on repo-root boundary."""
+
+from __future__ import annotations
+
+import os
+import sys
+from pathlib import Path
+
+import pytest
+
+sys.path.insert(0, str(Path(__file__).parent.parent))
+
+import check_cross_references
+
+
+@pytest.fixture
+def repo(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> Path:
+    monkeypatch.chdir(tmp_path)
+    return tmp_path
+
+
+def test_links_escaping_repo_root_are_skipped(
+    repo: Path, capsys: pytest.CaptureFixture[str]
+) -> None:
+    outside = repo.parent / "outside.md"
+    outside.write_text("# Outside")
+
+    rel = os.path.relpath(outside, repo)
+    (repo / "README.md").write_text(f"# Doc\n\n[escape]({rel})\n")
+
+    assert check_cross_references.main() == 0
+    assert "broken cross-reference" not in capsys.readouterr().out
+
+
+def test_broken_in_repo_link_is_reported(
+    repo: Path, capsys: pytest.CaptureFixture[str]
+) -> None:
+    (repo / "README.md").write_text("# Doc\n\n[missing](does-not-exist.md)\n")
+
+    assert check_cross_references.main() == 1
+    assert "broken cross-reference" in capsys.readouterr().out
+
+
+def test_valid_in_repo_link_passes(
+    repo: Path, capsys: pytest.CaptureFixture[str]
+) -> None:
+    (repo / "other.md").write_text("# Other")
+    (repo / "README.md").write_text("# Doc\n\n[ok](other.md)\n")
+
+    assert check_cross_references.main() == 0
+    assert "All cross-references valid" in capsys.readouterr().out
+
+
+def test_numbered_lesson_dir_missing_readme_is_reported(
+    repo: Path, capsys: pytest.CaptureFixture[str]
+) -> None:
+    (repo / "README.md").write_text("# Doc")
+    (repo / "01-intro").mkdir()
+
+    assert check_cross_references.main() == 1
+    assert "01-intro: missing README.md" in capsys.readouterr().out