feat(embeddings): every vector records the model whose space it lives in (#4132)

The four embedding tables stamped chunker_version but not the model, and
vector(384) is a width, not an identity: a same-width model swap would
write a second geometry beside the first with no error.

- embedding_model on note/rule/milestone/system embeddings (0109; existing
  rows stamped with the only model any install has ever run).
- Every write stamps EMBEDDING_MODEL; every backfill's "current" test is
  is_current_stamp(), both halves of calibration_stamp().
- migrate_floor refuses while any row its surface searches is off the live
  model, before sampling: re-embed, then migrate.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_integration_milestone_search.py

@@ -17,7 +17,7 @@ from scribe.models.embedding import EMBEDDING_DIM, MilestoneEmbedding
 from scribe.models.milestone import Milestone
 from scribe.models.project import Project
 from scribe.services import dedup as dedup_svc
-from scribe.services.embeddings import CHUNKER_VERSION, semantic_search_milestones
+from scribe.services.embeddings import CHUNKER_VERSION, EMBEDDING_MODEL, semantic_search_milestones
 from tests.helpers import ensure_user
 
 pytestmark = [pytest.mark.integration, pytest.mark.usefixtures("_dispose_engine", "_no_embedding")]
@@ -48,7 +48,8 @@ async def roadmap():
         await s.flush()
         for ms, vec in ((m3, NEAR), (done, NEAR), (unrelated, FAR), (foreign, NEAR)):
             s.add(MilestoneEmbedding(milestone_id=ms.id, chunk_index=0, embedding=vec,
-                                     chunk_text=ms.title, chunker_version=CHUNKER_VERSION))
+                                     chunk_text=ms.title, chunker_version=CHUNKER_VERSION,
+                                     embedding_model=EMBEDDING_MODEL))
         ids = {"owner": owner.id, "stranger": stranger.id, "mine": mine.id,
                "m3": m3.id, "done": done.id, "unrelated": unrelated.id, "foreign": foreign.id}
         await s.commit()