fix(verify-manual): use a real temp file instead of /dev/stdin for the registry round-trip test

fish 3.7 (Ubuntu 24.04's packaged version, used in CI) rejects
'source /dev/stdin' when it's backed by a pipe, with
"'/dev/stdin' is not a file" -- fish 4.8 (local dev) accepts it.
Writing the generated registry to a real temp file sources
identically across fish versions.

docs/verify-manual.py

@@ -1097,8 +1097,16 @@ def test_render_registry_is_valid_fish_and_round_trips():
     """Sourcing render()'s output must leave the two arrays in the exact
     shape __fish_config_op_registry_lookup expects -- checked via the real
     lookup helper (functions/__fish_config_op_registry_lookup.fish, Task
-    2) rather than re-parsing the generated text by hand."""
+    2) rather than re-parsing the generated text by hand.
+
+    Written to a real temp file rather than piped through `source
+    /dev/stdin`: fish 3.7 (Ubuntu 24.04's packaged version, used in CI)
+    rejects `/dev/stdin` with "is not a file" when it's backed by a pipe,
+    even though fish 4.8 accepts it -- sourcing an actual file on disk is
+    the portable form across fish versions.
+    """
     import subprocess
+    import tempfile
 
     import generate_component_registry as gcr
 
@@ -1109,14 +1117,16 @@ def test_render_registry_is_valid_fish_and_round_trips():
     text = gcr.render(registry)
 
     repo = Path(__file__).parent.parent
+    with tempfile.TemporaryDirectory() as d:
+        registry_path = Path(d) / "registry.fish"
+        registry_path.write_text(text)
         proc = subprocess.run(
             [
                 "fish", "-c",
                 f"source {repo}/functions/__fish_config_op_registry_lookup.fish; "
-            "source /dev/stdin; "
+                f"source {registry_path}; "
                 "__fish_config_op_registry_lookup rm ''; echo status=$status",
             ],
-        input=text,
             capture_output=True,
             text=True,
         )