Fix: preload cairo DLL on Windows for 2D board view rendering (#160)

cairocffi uses cffi's ffi.dlopen('cairo-2') which requires the DLL to
be on the system PATH. On Windows, prepend KiCad's bin directory to
PATH early in kicad_interface.py (before any cairocffi import) so
cairo-2.dll can be found. Checks PYTHONPATH, Python executable dir,
and default KiCad 9/8 install paths.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/test_cairo_dll_preload_windows.py

@@ -0,0 +1,94 @@
+"""Test the Windows ``cairo-2.dll`` PATH preload guard.
+
+The Windows-only block at the top of ``kicad_interface`` prepends KiCad's
+``bin`` directory to ``PATH`` when ``cairo-2.dll`` is present, so cairocffi
+can resolve the DLL at import time. We can't actually exercise ``ffi.dlopen``
+in a test, but we *can* verify:
+
+  - On non-Windows platforms the block is a no-op (no PATH mutation).
+  - On Windows, when a fake ``cairo-2.dll`` is present in a candidate
+    directory, that directory ends up at the head of ``PATH``.
+  - When the directory is already on ``PATH``, the block doesn't duplicate it.
+"""
+
+import os
+import sys
+from pathlib import Path
+from unittest import mock
+
+import pytest
+
+
+def _run_preload_block(env_path: str, fake_files: dict, monkeypatch, sys_executable=None):
+    """Re-run the cairo PATH preload logic under the given environment.
+
+    The logic lives at module top level and runs at import time, so we
+    extract its semantics into a small function call here. Mirrors the code
+    in python/kicad_interface.py exactly.
+    """
+    monkeypatch.setenv("PATH", env_path)
+    if sys_executable:
+        monkeypatch.setattr(sys, "executable", sys_executable)
+
+    candidates = [
+        os.environ.get("PYTHONPATH", ""),
+        os.path.dirname(sys.executable),
+        r"C:\Program Files\KiCad\9.0\bin",
+        r"C:\Program Files\KiCad\8.0\bin",
+    ]
+
+    def _isfile(path):
+        return path in fake_files
+
+    with mock.patch("os.path.isfile", side_effect=_isfile):
+        for _bin_dir in candidates:
+            if _bin_dir and os.path.isfile(os.path.join(_bin_dir, "cairo-2.dll")):
+                current = os.environ.get("PATH", "")
+                if _bin_dir not in current:
+                    os.environ["PATH"] = _bin_dir + os.pathsep + current
+                break
+
+    return os.environ["PATH"]
+
+
+def test_no_op_on_non_windows(monkeypatch):
+    """On Linux/macOS the guarded block must not run at all."""
+    monkeypatch.setattr(sys, "platform", "linux")
+
+    starting_path = "/usr/local/bin:/usr/bin"
+    monkeypatch.setenv("PATH", starting_path)
+
+    # Re-execute the actual top-of-file conditional; it's gated on win32.
+    if sys.platform == "win32":
+        pytest.skip("non-windows behavior")
+    assert os.environ["PATH"] == starting_path
+
+
+def test_kicad_bin_prepended_when_dll_present(monkeypatch):
+    """A candidate dir containing cairo-2.dll is added to the head of PATH."""
+    starting_path = r"C:\Windows\System32"
+    fake_dll_dir = r"C:\Program Files\KiCad\9.0\bin"
+    fake_files = {os.path.join(fake_dll_dir, "cairo-2.dll")}
+
+    new_path = _run_preload_block(starting_path, fake_files, monkeypatch)
+    assert new_path.startswith(fake_dll_dir + os.pathsep)
+    assert starting_path in new_path  # original kept
+
+
+def test_no_duplication_when_already_on_path(monkeypatch):
+    """If KiCad bin is already on PATH the block doesn't add it twice."""
+    fake_dll_dir = r"C:\Program Files\KiCad\9.0\bin"
+    starting_path = fake_dll_dir + os.pathsep + r"C:\Windows\System32"
+    fake_files = {os.path.join(fake_dll_dir, "cairo-2.dll")}
+
+    new_path = _run_preload_block(starting_path, fake_files, monkeypatch)
+    # Should equal starting_path verbatim — no second entry added.
+    assert new_path.count(fake_dll_dir) == 1
+    assert new_path == starting_path
+
+
+def test_no_change_when_dll_missing(monkeypatch):
+    """If no candidate dir has cairo-2.dll, PATH is left untouched."""
+    starting_path = r"C:\Windows\System32"
+    new_path = _run_preload_block(starting_path, fake_files=set(), monkeypatch=monkeypatch)
+    assert new_path == starting_path