test(extractors): guard facade + registry identity for the per-language split (#1721)

The extract_terraform move #1721 proposed already landed on v8 via the #1737
decomposition (extractors/terraform.py exists, extract.py re-exports it, and
extractors/LANGUAGE_EXTRACTORS registers it), so the code move is a no-op now.
But the regression test @Cekaru added with it had no equivalent on v8. Salvage
and generalize it: sweep every LANGUAGE_EXTRACTORS entry and assert graphify.
extract re-exports the SAME object (facade identity) and the registry maps to
it (registry identity), plus the concrete terraform anchor from the PR. This
institutionalizes the re-export-identity guarantee the split relies on, so a
future move that forgets a facade re-export fails loudly.

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

tests/test_extractors_registry.py

@@ -0,0 +1,44 @@
+"""Facade / registry identity guards for the per-language extractor split (#1212).
+
+The ``extract.py`` decomposition (#1737) moved each language extractor into its
+own ``graphify/extractors/<lang>.py`` module, kept a verbatim re-export in
+``graphify.extract`` (the facade every existing importer uses), and seeded a
+``graphify.extractors.LANGUAGE_EXTRACTORS`` registry. Three things must stay
+true for that split to be behavior-preserving:
+
+- the function is importable from its new per-language module,
+- ``graphify.extract`` still re-exports the SAME function object (facade
+  identity — a stale copy or shadowing import would silently diverge),
+- ``LANGUAGE_EXTRACTORS`` maps to that same object (registry identity).
+
+Originally proposed by @Cekaru in #1721 as a per-language check; generalized
+here to sweep the whole registry so a future move that forgets the facade
+re-export (or re-exports a different object) fails loudly.
+"""
+from __future__ import annotations
+
+import graphify.extract as facade
+from graphify.extractors import LANGUAGE_EXTRACTORS
+
+
+def test_every_registry_extractor_is_reexported_from_facade():
+    missing = []
+    diverged = []
+    for lang, fn in LANGUAGE_EXTRACTORS.items():
+        name = getattr(fn, "__name__", None)
+        if not name or not hasattr(facade, name):
+            missing.append((lang, name))
+            continue
+        if getattr(facade, name) is not fn:
+            diverged.append((lang, name))
+    assert not missing, f"registry extractors not re-exported from graphify.extract: {missing}"
+    assert not diverged, f"facade object diverges from registry: {diverged}"
+
+
+def test_terraform_migrated():
+    # The concrete anchor from #1721: extract_terraform lives in its own module,
+    # and both the facade and the registry point at that one object.
+    from graphify.extractors.terraform import extract_terraform
+
+    assert facade.extract_terraform is extract_terraform
+    assert LANGUAGE_EXTRACTORS["terraform"] is extract_terraform