fix(telemetry): a floor that moved inside the window makes the band check a comparison of two populations (#4225)

`retrieval_telemetry(days=30)` reported, for write_path_rule:

  "the weakest tenth of what this arm returns scores 0.6984, only -0.0216
   above its floor of 0.72"

A negative distance above something. The tenth percentile of what an arm
RETURNED cannot sit below the floor that gates what it may return — not
inside one population.

MEASURED CAUSE. write_path_rule's floor was 0.68 until 2026-09-02, when
2385100 (#3318) raised the shipped default to 0.72. The window opened
2026-08-22, so six days of it are calls made under the old bar; top_score.min
for the surface is exactly 0.68, the old bar still in the sample.

AND THE CHANGE LEFT NO TRACE THE READOUT COULD SEE. retrieval_tuning_events
records dial turns — a person or a model choosing a number. It was silent
about the other way a floor moves: somebody edits floor_default and ships it.
retrieval_tuning_history returned {"events": []} and retrieval_surfaces said
last_change: {}, source: "shipped". All true, and all of it silent about a
floor that had in fact moved.

THE RAISE ANNOUNCED ITSELF. A LOWERED FLOOR WOULD NOT: the gap comes out
comfortably positive and reads as a clean bill of health on a sample that
half predates the bar being judged. Both directions are now pinned.

So the check is SUSPENDED, not softened. band_hugs_floor asks whether the
scores are piled on the bar; that needs the scores and the bar to come from
the same regime. Where they do not, the honest answer is that this sample
cannot say, plus the date after which one can — floor_moved_mid_window
replaces band_hugs_floor for that arm and never accompanies it. A reader told
a number is unavailable goes and gets one; a reader handed a qualified number
uses it.

NO MIGRATION. `actor` is Text with no CHECK precisely so a new kind of actor
is not one — the model's own comment says so, and this is the case it
anticipated. "release" joins "model" and "human". user_id is already
nullable, which is right: no user did this, a release acts on every account
that has not overridden the dial, and a row per user would both multiply and
misattribute it. Both readers now take the newest of (this user's change, the
release's).

THE FIRST SIGHTING IS A BASELINE, written with old_value NULL. Nothing moved;
the row exists so the next release has a predecessor. That null is
load-bearing: floor_moves_since asks for old_value IS NOT NULL, so a fresh
install's baseline does not silently retire the check on every new install.

UI: the tuning history rendered actor as `human ? 'you' : 'Claude'`, so a
release row would have told the operator that Claude moved a floor it never
touched — the one failure the actor column exists to prevent. Three-way now,
with an unknown value printing itself rather than guessing.

Recorded at startup, inline and awaited. What #4181 cost three hours was
concurrency — a background task racing the hook for the same pool. Sequential
creates no contention, and this is twelve single-row reads. It must finish
before serving because a readout served before the change was recorded is the
exact answer this exists to stop giving.

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

src/scribe/services/retrieval_telemetry.py

@@ -40,6 +40,7 @@ from scribe.services.retrieval_registry import (
     POINTS, UNBIDDEN, get_point, is_registered, sources_expected_to_emit,
 )
 from scribe.services.retrieval_surfaces import SURFACES, floor_for
+from scribe.services.retrieval_tuning import floor_moves_since
 from scribe.services.settings import get_setting
 
 logger = logging.getLogger(__name__)
@@ -498,7 +499,8 @@ def _warn(code, detail, source=None, **numbers) -> dict:
 
 
 def _compute_warnings(sources: dict, usage: dict, rule_usage: dict,
-                      floors: dict, min_calls: int, epsilon: float) -> list[dict]:
+                      floors: dict, min_calls: int, epsilon: float,
+                      floor_moves: dict | None = None) -> list[dict]:
     """The four checks, over whatever sources the window actually contains.
 
     DELIBERATELY NOT KEYED ON A HARD-CODED SOURCE LIST. An arm added next
@@ -581,9 +583,41 @@ def _compute_warnings(sources: dict, usage: dict, rule_usage: dict,
         # invite tuning a dial that does not exist.
         floor = floors.get(name)
         p10 = (b.get("top_score") or {}).get("p10")
+        moved = (floor_moves or {}).get(name)
         if calls >= min_calls and floor is not None and p10 is not None:
-            gap = p10 - floor
-            if gap < epsilon:
+            # ── The floor moved inside the window ────────────────────────
+            #
+            # THE CHECK IS SUSPENDED, NOT SOFTENED, and this is the whole of
+            # #4225. `band_hugs_floor` asks whether the scores are piled on
+            # the bar. That question needs the scores and the bar to come from
+            # the same regime; across a floor change they do not, and the
+            # comparison quietly becomes one between two populations.
+            #
+            # It announced itself when the change was a RAISE: p10 computed
+            # over calls made under the old, lower bar came out BELOW today's
+            # floor, and the warning reported a band "-0.0216 above" its
+            # floor. A negative distance above something is not a number
+            # anybody can act on. A LOWERED floor hides better — the gap comes
+            # out comfortably positive and reads as a clean bill of health on
+            # a sample that half predates the bar being judged.
+            #
+            # So the honest move is to say the sample cannot answer, and say
+            # when it will be able to, rather than print a figure with an
+            # asterisk. A reader who is told a number is unavailable goes and
+            # gets one; a reader handed a qualified number uses it.
+            if moved is not None:
+                out.append(_warn(
+                    "floor_moved_mid_window",
+                    f"this arm's floor changed at {moved}, inside the window, "
+                    f"so its calls were made under two different bars and its "
+                    f"band cannot be compared against the floor now in force "
+                    f"({floor}). The band check is suspended for this arm "
+                    f"until the window clears that date — ask again with a "
+                    f"`days` that starts after it, or wait.",
+                    source=name, floor=floor, p10=p10, moved_at=moved,
+                ))
+            elif p10 - floor < epsilon:
+                gap = p10 - floor
                 out.append(_warn(
                     "band_hugs_floor",
                     f"the weakest tenth of what this arm returns scores "
@@ -1350,8 +1384,20 @@ async def retrieval_summary(
                 except Exception:  # pragma: no cover - telemetry never raises
                     logger.warning("could not read floor for %s", name, exc_info=True)
 
+    # Floors that MOVED inside this window (#4225) — a release's shipped
+    # default or the operator's own dial, because the question is about the
+    # sample, not about who is answerable for it. Read unconditionally rather
+    # than under `if user_id`, unlike `floors` above: a release change belongs
+    # to no account, and it is exactly the case that used to go unrecorded.
+    floor_moves: dict[str, str] = {}
+    try:
+        floor_moves = await floor_moves_since(since)
+    except Exception:  # pragma: no cover - telemetry never raises
+        logger.warning("could not read floor changes", exc_info=True)
+
     out["warnings"] = _compute_warnings(
         out["sources"], usage, rule_usage, floors, min_calls, epsilon,
+        floor_moves,
     )
 
     # "Active" means this window saw real traffic SOMEWHERE. Without that