runner: never embed a fossil screenshot from a collided run dir (#45)

tests/unit/test_results.py

@@ -489,3 +489,50 @@ def test_write_results_roundtrip(tmp_path):
     assert path.endswith("/42/results.json")
     with open(path) as f:
         assert json.load(f)["level"] == 3
+
+
+def test_fresh_artifact_dir_removes_owned_stale_artifacts(tmp_path):
+    """Run-dir collision guard (2026-10-05): drone build numbers restarted while old-era run dirs
+    linger in /var/lib/cc-ci-runs — a new build reusing an old number must not mix the old run's
+    artifacts (fossil screenshot.png from ANOTHER recipe) into its card. Only OWNED artifacts go."""
+    d = tmp_path / "35"
+    d.mkdir()
+    (d / "results.json").write_text('{"old": true}')
+    (d / "screenshot.png").write_bytes(b"stale-fossil")
+    (d / "summary.png").write_bytes(b"stale")
+    (d / "badge.svg").write_text("<svg/>")
+    (d / "lint.txt").write_text("old")
+    (d / "summary.html").write_text("<html/>")
+    (d / "junit").mkdir()
+    (d / "junit" / "old.xml").write_text("<x/>")
+    (d / "untracked-thing.bin").write_bytes(b"keep me")  # NOT owned — must survive
+
+    R.fresh_artifact_dir(str(d))
+
+    for name in (
+        "results.json",
+        "screenshot.png",
+        "summary.png",
+        "badge.svg",
+        "lint.txt",
+        "summary.html",
+    ):
+        assert not (d / name).exists(), f"stale owned artifact {name} survived"
+    assert not (d / "junit").exists(), "stale junit dir survived"
+    assert (d / "untracked-thing.bin").read_bytes() == b"keep me", "unowned file was deleted"
+
+
+def test_fresh_artifact_dir_creates_missing_dir(tmp_path):
+    d = tmp_path / "36"
+    out = R.fresh_artifact_dir(str(d))
+    assert out == str(d) and d.is_dir()
+
+
+def test_fresh_artifact_dir_tolerant_of_odd_entries(tmp_path):
+    """R7: never raise into the run — a dir where an owned name is itself a dir must still work."""
+    d = tmp_path / "37"
+    d.mkdir()
+    (d / "screenshot.png").mkdir()  # perverse: a DIRECTORY named screenshot.png
+    (d / "junit").mkdir()
+    R.fresh_artifact_dir(str(d))
+    assert not (d / "screenshot.png").exists() and not (d / "junit").exists()