feat(gui): échec amont clair si dossier de sortie non inscriptible (P1-6)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/unit/test_gui_v6_processing_runner.py

@@ -197,6 +197,23 @@ def test_progress_callbacks(tmp_path):
     assert (2, 2) in events  # progression finale atteinte
 
 
+def test_run_fails_fast_when_output_not_writable(tmp_path, monkeypatch):
+    from gui_v6.processing_runner import ProcessingRunner, OutputNotWritableError
+    src = tmp_path / "in"
+    src.mkdir()
+    (src / "a.txt").write_text("x", encoding="utf-8")
+    out = tmp_path / "ro"
+    out.mkdir()
+
+    def boom(*a, **k):
+        raise PermissionError("read-only")
+
+    monkeypatch.setattr("gui_v6.processing_runner.Path.mkdir", boom)
+    runner = ProcessingRunner(process_fn=lambda d, o: {})
+    with pytest.raises(OutputNotWritableError):
+        runner.run(src, out)
+
+
 def test_no_double_run(tmp_path):
     _touch(tmp_path / "a.pdf")
     started = threading.Event()