fix(illness): gate the recovery-gap floor on adverse-direction days

The cross-episode typical-gap median counts an episode toward its
qualifying-days floor via the per-episode coverage `historyDays`, which
counts a day if ANY banded vital has a reading. A WEIGHT-only episode (no
illness-adverse direction) could therefore clear the floor and feed the
median despite carrying no illness-relevant signal, weakening the honest-
sample claim.

Compute a distinct-adverse-day count in the engine — days with at least
one notable reading in the metric's adverse direction — and feed that as
the qualifying-days floor instead. Reuses the engine's single adverse-
direction predicate (now exported as `isAdverseDeviation`); a neutral-
direction vital never counts a day. The insufficient arm contributes 0,
which is correct since its gap is null and never reaches the median.

docs/api/openapi.yaml

@@ -20279,6 +20279,10 @@ components:
           anyOf:
             - type: number
             - type: "null"
+        adverseCoverageDays:
+          type: integer
+          minimum: -9007199254740991
+          maximum: 9007199254740991
         feltBetterDay:
           anyOf:
             - type: string
@@ -20293,6 +20297,7 @@ components:
         - nadir
         - returns
         - recoveryGapDays
+        - adverseCoverageDays
         - feltBetterDay
         - redFlags
       additionalProperties: false