fix(filesystem): lazy-load pifs import surface

Avoid eager optional dependency imports when importing PageIndexFileSystem or filesystem semantic exports.

tests/test_import_surface.py

@@ -0,0 +1,35 @@
+from __future__ import annotations
+
+import builtins
+import importlib
+import sys
+
+
+def test_filesystem_import_works_without_eager_optional_dependencies(monkeypatch):
+    blocked_roots = {"litellm", "openai", "PyPDF2", "pymupdf", "sqlite_vec"}
+    real_import = builtins.__import__
+
+    def clear_pageindex_modules() -> None:
+        for name in list(sys.modules):
+            if name == "pageindex" or name.startswith("pageindex."):
+                sys.modules.pop(name, None)
+
+    def import_without_optional_deps(name, globals=None, locals=None, fromlist=(), level=0):
+        root = name.split(".", 1)[0]
+        if root in blocked_roots:
+            raise ModuleNotFoundError(f"No module named '{root}'", name=root)
+        return real_import(name, globals, locals, fromlist, level)
+
+    clear_pageindex_modules()
+    try:
+        with monkeypatch.context() as patch:
+            patch.setattr(builtins, "__import__", import_without_optional_deps)
+
+            filesystem_module = importlib.import_module("pageindex.filesystem")
+            from pageindex import PageIndexFileSystem as TopLevelPageIndexFileSystem
+            from pageindex.filesystem import PageIndexFileSystem
+
+            assert filesystem_module.PageIndexFileSystem is PageIndexFileSystem
+            assert TopLevelPageIndexFileSystem is PageIndexFileSystem
+    finally:
+        clear_pageindex_modules()