fix(filesystem): detect ambiguous virtual paths

Resolve root virtual file paths correctly and raise a clear error for ambiguous file targets.

tests/test_pifs_path_resolution.py

@@ -0,0 +1,44 @@
+import pytest
+
+
+def test_root_virtual_file_path_resolves_without_double_slash(tmp_path):
+    from pageindex.filesystem import PageIndexFileSystem
+
+    filesystem = PageIndexFileSystem(workspace=tmp_path / "workspace")
+    file_ref = filesystem.register_file(
+        storage_uri="file:///tmp/root-source.txt",
+        source_path="sources/root-source.txt",
+        folder_path="/",
+        external_id="doc_root_title",
+        title="Root Title",
+        content="root content",
+    )
+
+    assert filesystem.store.resolve_file_ref("/Root Title") == file_ref
+
+
+def test_ambiguous_virtual_file_path_raises_clear_error(tmp_path):
+    from pageindex.filesystem import PageIndexFileSystem
+
+    filesystem = PageIndexFileSystem(workspace=tmp_path / "workspace")
+    first_ref = filesystem.register_file(
+        storage_uri="file:///tmp/first.txt",
+        source_path="b/file.txt",
+        folder_path="/a",
+        external_id="doc_first",
+        title="First",
+        content="first content",
+    )
+    second_ref = filesystem.register_file(
+        storage_uri="file:///tmp/second.txt",
+        source_path="second-source.txt",
+        folder_path="/a/b",
+        external_id="doc_second",
+        title="file.txt",
+        content="second content",
+    )
+
+    with pytest.raises(KeyError, match="Ambiguous file target"):
+        filesystem.store.resolve_file_ref("/a/b/file.txt")
+
+    assert first_ref != second_ref