fix(arxiv): handle legacy single-file gzipped source (no tar wrapper)

Old arXiv submissions (e.g. legacy math/* papers) are served as a bare gzipped .tex rather than a .tar.gz, which tarfile.open rejected with 'invalid header'. Fall back to gunzipping the body directly when the tar parse fails, so R-C covers those papers too (recovers math/0001176 + math/0306167). Adds a regression test.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/sources/test_arxiv.py

@@ -86,6 +86,30 @@ def test_fetch_source_flattens_multifile(monkeypatch: pytest.MonkeyPatch) -> Non
     assert "\\input" not in result and "\\include" not in result
 
 
+def test_fetch_source_bare_gzip_single_file(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Legacy arXiv submissions are served as a single bare gzipped .tex (no tar
+    wrapper); fetch_source must gunzip the body directly rather than fail."""
+    import gzip
+
+    latex = b"\\documentstyle[a4]{article}\n\\section{Intro}\nOld single-file paper body."
+    gz_bytes = gzip.compress(latex)
+
+    def mock_get(
+        url: str,
+        *,
+        timeout: int = 60,
+        follow_redirects: bool = True,
+    ) -> httpx.Response:
+        return httpx.Response(200, content=gz_bytes)
+
+    monkeypatch.setattr(httpx, "get", mock_get)
+
+    result = arxiv.fetch_source("math/0001176")
+
+    assert result is not None
+    assert "Old single-file paper body." in result
+
+
 # ---------------------------------------------------------------------------
 # fetch_source — 404 → None
 # ---------------------------------------------------------------------------