test(dedup): stamp _origin on the shrink-guard fixture (post-#3110)

#3110 makes the semantic tier carry _origin:"semantic" explicitly, so a real
graph.json's semantic nodes are no longer tier-guessed from the L-shape of
source_location. This fixture wrote a baseline graph with no _origin, which
backfilled to AST and mixed with the re-extracted (now semantic) nodes, so the
pair no longer merged and the shrink guard had nothing to catch. Stamp the
fixture nodes semantic to reflect what real post-#3110 graphs contain; the guard
correctly fires again.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_dedup_shrink_refuses_force_write.py

@@ -23,6 +23,7 @@ def _notes(corpus):
             "file_type": "document",
             "source_file": readme,
             "source_location": "L1",
+            "_origin": "semantic",
         },
         {
             "id": "readme_decisions_b",
@@ -30,6 +31,7 @@ def _notes(corpus):
             "file_type": "document",
             "source_file": readme,
             "source_location": "L2",
+            "_origin": "semantic",
         },
         {
             "id": "readme_overview",
@@ -37,6 +39,7 @@ def _notes(corpus):
             "file_type": "document",
             "source_file": readme,
             "source_location": "L3",
+            "_origin": "semantic",
         },
     ]
 
@@ -144,6 +147,7 @@ def test_a_deleted_file_still_writes(monkeypatch, tmp_path):
             "file_type": "document",
             "source_file": readme,
             "source_location": "L1",
+            "_origin": "semantic",
         },
         {
             "id": "gone_notes",
@@ -151,6 +155,7 @@ def test_a_deleted_file_still_writes(monkeypatch, tmp_path):
             "file_type": "document",
             "source_file": gone,
             "source_location": "L1",
+            "_origin": "semantic",
         },
     ]
     fresh = [nodes[0]]