Add negative control test for an ordinary ASCII path

Confirms the reconstructed prefix set for an ASCII path is byte for
byte identical to what the old ASCII only regex produced, so the fix
changes behavior only for paths the old code was already getting
wrong.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017qfdzgbA5KedGEjD1AayNh
(cherry picked from commit 97bd1f37f0)

tests/test_dedup.py

@@ -572,6 +572,15 @@ def test_korean_defining_file_wins_over_referencing_file(nodes):
     )
 
 
+def test_id_prefixes_ascii_path_unchanged():
+    """Negative control: an ordinary ASCII path's reconstructed prefixes must
+    be identical to what the old ASCII only regex produced."""
+    from graphify.dedup import _id_prefixes
+    assert _id_prefixes("docs/v1/api/README.md") == {
+        "readme", "api_readme", "v1_api_readme", "docs_v1_api_readme",
+    }
+
+
 def test_same_file_relabel_is_noted(capsys):
     """Two labels for one ID from one file: the loser's label is discarded, which is
     the one drop that used to be silent. It is a note, not a collision warning."""