flake: prevent outPath in multiselect to avoid serialization issues

When using multiselect with outPath like {outPath,?meta}, nix evaluation
collapses the attrset to just the outPath string, breaking further selection.
Add validation during selector parsing to catch this and provide a clear error.

pkgs/clan-cli/clan_lib/flake/flake.py

@@ -168,6 +168,16 @@ def parse_selector(selector: str) -> list[Selector]:
                 else:
                     set_select_type = SetSelectorType.STR
                 acc_selectors.append(SetSelector(type=set_select_type, value=acc_str))
+                # Check for invalid multiselect patterns with outPath
+                for subselector in acc_selectors:
+                    if subselector.value == "outPath":
+                        msg = (
+                            "Cannot use 'outPath' in multiselect {...}. "
+                            "When nix evaluates attrsets with outPath in a multiselect, "
+                            "it collapses the entire attrset to just the outPath string, "
+                            "breaking further selection. Use individual selectors instead."
+                        )
+                        raise ValueError(msg)
                 selectors.append(Selector(type=SelectorType.SET, value=acc_selectors))
 
                 submode = ""

pkgs/clan-cli/clan_lib/flake/flake_test.py

@@ -168,6 +168,20 @@ def test_out_path() -> None:
     assert test_cache.select(selectors) == "/nix/store/bla"
 
 
+def test_out_path_in_multiselect_raises_exception() -> None:
+    with pytest.raises(ValueError, match="Cannot use 'outPath' in multiselect"):
+        parse_selector("{outPath}")
+
+    with pytest.raises(ValueError, match="Cannot use 'outPath' in multiselect"):
+        parse_selector("x.{y,outPath}")
+
+    with pytest.raises(ValueError, match="Cannot use 'outPath' in multiselect"):
+        parse_selector("a.b.{c,d,outPath,e}")
+
+    with pytest.raises(ValueError, match="Cannot use 'outPath' in multiselect"):
+        parse_selector("{?outPath}")
+
+
 @pytest.mark.with_core
 def test_conditional_all_selector(flake: ClanFlake) -> None:
     m1 = flake.machines["machine1"]