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>

src/scribe/models/embedding.py

@@ -41,6 +41,14 @@ class NoteEmbedding(Base):
     # any note whose rows carry a stale version — shape changes become a
     # version bump instead of a table wipe.
     chunker_version: Mapped[int] = mapped_column(Integer, nullable=False)
+    # embeddings.EMBEDDING_MODEL at write time — the SPACE the vector lives in
+    # (#4132). The column is `vector(384)`, a width and not an identity, so a
+    # swap to another 384-dim model writes a second geometry beside the first
+    # with no error, and cosine across the two is a number that means nothing.
+    # The version above says what text was embedded; this says in whose
+    # geometry. Either one moving makes the row stale, and the backfill
+    # re-embeds on both.
+    embedding_model: Mapped[str] = mapped_column(Text, nullable=False)
     updated_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True),
         default=lambda: datetime.now(timezone.utc),
@@ -91,6 +99,7 @@ class RuleEmbedding(Base):
     # centroid (measured in note 2485).
     chunk_text: Mapped[str] = mapped_column(Text, nullable=False)
     chunker_version: Mapped[int] = mapped_column(Integer, nullable=False)
+    embedding_model: Mapped[str] = mapped_column(Text, nullable=False)  # see NoteEmbedding
     updated_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True),
         default=lambda: datetime.now(timezone.utc),
@@ -124,6 +133,7 @@ class MilestoneEmbedding(Base):
     embedding: Mapped[list] = mapped_column(Vector(EMBEDDING_DIM), nullable=False)
     chunk_text: Mapped[str] = mapped_column(Text, nullable=False)
     chunker_version: Mapped[int] = mapped_column(Integer, nullable=False)
+    embedding_model: Mapped[str] = mapped_column(Text, nullable=False)  # see NoteEmbedding
     updated_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True),
         default=lambda: datetime.now(timezone.utc),
@@ -163,6 +173,7 @@ class SystemEmbedding(Base):
     embedding: Mapped[list] = mapped_column(Vector(EMBEDDING_DIM), nullable=False)
     chunk_text: Mapped[str] = mapped_column(Text, nullable=False)
     chunker_version: Mapped[int] = mapped_column(Integer, nullable=False)
+    embedding_model: Mapped[str] = mapped_column(Text, nullable=False)  # see NoteEmbedding
     updated_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True),
         default=lambda: datetime.now(timezone.utc),