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.

src/lib/openapi/routes/illness.ts

@@ -208,6 +208,7 @@ const illnessCorrelationValue = z
     nadir: z.array(illnessVitalDeviation),
     returns: z.array(illnessVitalReturn),
     recoveryGapDays: z.number().nullable(),
+    adverseCoverageDays: z.number().int(),
     feltBetterDay: z.string().nullable(),
     redFlags: z.array(illnessRedFlag),
   })