refactor: use pathlib for file paths in new export handlers

CONTRIBUTING.md mandates pathlib.Path over os.path (added for
cross-platform/Linux support); the rest of the codebase's newer code
already follows it. Converts the 19 new _handle_export_* handlers to
pathlib (exists/mkdir/iterdir/is_file, expanduser().resolve()) so this
feature's new code is compliant. Tests updated to patch pathlib.Path.

Scope is limited to the new export-handler block; the pre-existing
os.path usage elsewhere in kicad_interface.py/export.py is untouched
here and will be migrated in a dedicated refactor PR to keep that
sweep reviewable in isolation.

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

tests/test_export_cli.py

@@ -64,10 +64,10 @@ def _call(iface, suffix, params, rc=0, stderr=""):
     fake = SimpleNamespace(returncode=rc, stdout="", stderr=stderr)
     with (
         patch("subprocess.run", return_value=fake) as run,
-        patch("os.path.exists", return_value=True),
-        patch("os.makedirs"),
-        patch("os.listdir", return_value=["a.out"]),
-        patch("os.path.isfile", return_value=True),
+        patch("pathlib.Path.exists", return_value=True),
+        patch("pathlib.Path.mkdir"),
+        patch("pathlib.Path.iterdir", return_value=[]),
+        patch("pathlib.Path.is_file", return_value=True),
     ):
         result = method(dict(params))
     return result, run