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_lesson_reach.py

@@ -23,7 +23,7 @@ from scribe.models.embedding import EMBEDDING_DIM, NoteEmbedding
 from scribe.models.note import Note
 from scribe.models.project import Project
 from scribe.services import lessons as lessons_svc
-from scribe.services.embeddings import CHUNKER_VERSION, semantic_search_notes
+from scribe.services.embeddings import CHUNKER_VERSION, EMBEDDING_MODEL, semantic_search_notes
 from tests.helpers import ensure_user
 
 pytestmark = [pytest.mark.integration, pytest.mark.usefixtures("_dispose_engine")]
@@ -71,6 +71,7 @@ async def corpus():
                 note_id=note.id, chunk_index=0, user_id=owner.id,
                 embedding=QUERY_VEC, chunk_text=note.title,
                 chunker_version=CHUNKER_VERSION,
+                embedding_model=EMBEDDING_MODEL,
             ))
         ids = {k: n.id for k, n in rows.items()}
         ids["owner"], ids["a"], ids["b"] = owner.id, a.id, b.id