fix: include KiCad 10 Windows install paths in library auto-discovery

Fixes #245 — _find_kicad_footprint_dir() only listed the 9.0/8.0 Windows
install paths, so a Windows host with only KiCad 10 installed got None and
footprint auto-discovery found nothing. The same gap existed for symbols in
library_symbol.py and dynamic_symbol_loader.py.

- Add C:/Program Files/KiCad/10.0/share/kicad/{footprints,symbols} to the
  candidate lists (10.0 first, matching the existing newest-first order)
- Honor KICAD10_FOOTPRINT_DIR and KICAD10_SYMBOL_DIR env overrides alongside
  the existing 9/8 ones
- Regression tests in tests/test_kicad10_paths.py

tests/test_kicad10_paths.py

@@ -0,0 +1,46 @@
+"""Regression tests for issue #245: KiCad 10 Windows install paths must be in
+the auto-discovery candidate lists (footprints and symbols), with env-var
+overrides for KiCad 10.
+"""
+
+import sys
+from pathlib import Path
+from unittest.mock import patch
+
+import pytest
+
+sys.path.insert(0, str(Path(__file__).parent.parent / "python"))
+
+from commands.library import LibraryManager  # noqa: E402
+from commands.library_symbol import SymbolLibraryManager  # noqa: E402
+
+K10_FOOTPRINTS = "C:/Program Files/KiCad/10.0/share/kicad/footprints"
+K10_SYMBOLS = "C:/Program Files/KiCad/10.0/share/kicad/symbols"
+
+
+@pytest.mark.unit
+class TestKicad10FootprintDir:
+    def test_k10_windows_path_is_discovered(self):
+        lm = LibraryManager.__new__(LibraryManager)
+        with patch("commands.library.os.path.isdir", side_effect=lambda p: p == K10_FOOTPRINTS):
+            assert lm._find_kicad_footprint_dir() == K10_FOOTPRINTS
+
+    def test_k10_env_override_wins(self, monkeypatch):
+        lm = LibraryManager.__new__(LibraryManager)
+        monkeypatch.setenv("KICAD10_FOOTPRINT_DIR", "/custom/k10/footprints")
+        with patch("commands.library.os.path.isdir", return_value=True):
+            assert lm._find_kicad_footprint_dir() == "/custom/k10/footprints"
+
+
+@pytest.mark.unit
+class TestKicad10SymbolDir:
+    def test_k10_windows_path_is_discovered(self):
+        sm = SymbolLibraryManager.__new__(SymbolLibraryManager)
+        with patch("commands.library_symbol.os.path.isdir", side_effect=lambda p: p == K10_SYMBOLS):
+            assert sm._find_kicad_symbol_dir() == K10_SYMBOLS
+
+    def test_k10_env_override_wins(self, monkeypatch):
+        sm = SymbolLibraryManager.__new__(SymbolLibraryManager)
+        monkeypatch.setenv("KICAD10_SYMBOL_DIR", "/custom/k10/symbols")
+        with patch("commands.library_symbol.os.path.isdir", return_value=True):
+            assert sm._find_kicad_symbol_dir() == "/custom/k10/symbols"