feat(retrieval): a tuned number carries the space it was measured in (#4104)

Milestone 416 step 6. A retrieval floor is a cosine similarity, which only
means something inside one embedding model's geometry over documents cut one
particular way. Change either and every floor on the install keeps applying
while describing nothing — and nothing anywhere says so, because the scores
simply come out different and the bar goes on cutting.

`CHUNKER_VERSION` already solved this for documents: stamped per row, so the
backfill re-embeds precisely what is stale. The same idea, applied to the
numbers:

- `calibration_stamp()` — embedding model + document shape, one definition.
  TWO fields, never a fused string (rule 149): a mismatch has to say WHICH half
  moved, because they call for different responses.
- `retrieval_tuning_events` gains `embedding_model` / `shape_version`
  (migration 0104), stamped on every write. Nullable and NOT backfilled —
  "unstamped" is the honest answer for a row written before this existed, and
  it reports as `stale: null`, never as fine.
- `current_settings` reports calibration per dial: tuned rows from their event,
  untouched dials from the registry default's own stamp.
- `retrieval_surfaces` and the Settings panel show the mismatch. The panel
  renders ONLY when something is stale, so seeing it at all is the signal.
- `migrate_floor` / `migrate_retrieval_floor` answers "a path for thresholds to
  be inherited by the next model so that they don't have to recalibrate a lot":
  the raw cosine cannot cross models, but the PERCENTILE it represented can.
  Measure what fraction of a surface's logged calls the old floor admitted,
  re-score those queries under the current model, take the value admitting the
  same fraction. Dry run by default; applying writes an ordinary tuning event
  with the arithmetic in its reason.

Nothing auto-retunes. A stale stamp says a number is no longer a measurement;
it does not say what the number should be, and #4102 measured the one case
where the statistic and the correct action pointed opposite ways.

The load-bearing test is an ABSENCE: no chat-model identifier may appear
anywhere in the calibration path. Claude produces none of these scores, so a
Claude upgrade must trigger nothing — a false alarm here teaches the operator
to ignore the real one on the day bge-small becomes bge-base.

Backup v17 carries both columns, unfilled on the way out and on the way back:
a round trip must not turn "we don't know" into a stated fact.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

src/scribe/services/retrieval_surfaces.py

@@ -103,6 +103,31 @@ class Surface:
     asks: str
     over: str
     fires: str
+    measured_model: str = "BAAI/bge-small-en-v1.5"
+    measured_shape: int = 1
+    """What the SHIPPED defaults above were measured against (#4104).
+
+    A floor is a distance in one embedding model's geometry, over documents cut
+    one particular way. Either can change, and when one does every number in
+    this table describes something that no longer exists.
+
+    TWO FIELDS, NEVER ONE FUSED STRING (rule 149). A mismatch has to be able to
+    say WHICH half moved: a new embedding model and a re-cut document shape
+    invalidate the same numbers for different reasons and call for different
+    responses. `"<model>@<n>"` could only report that something changed, which
+    is the answer nobody can act on. Same reason `calibration_stamp()` returns
+    a dict and the event table gives each half its own column.
+
+    Recorded per surface rather than once for the module because they need not
+    move together: a surface retuned after a model change carries the new stamp
+    while its untouched siblings still carry the old one, and telling those
+    apart is the whole job.
+
+    LITERALS, deliberately, rather than an import of the live values — a stamp
+    says what was true when the number was chosen, so one that tracked the
+    current model would always agree with it and could never report staleness.
+    """
+
     budget_falls_back_to: str = ""
     """A budget key to inherit when this surface has none of its own set.