feat(illness): name the driving vital in the recovery-gap summary

The cross-episode summary already computes per-vital physiological returns
but the card surfaced only one bare line. Thread the dominant returning
vital through the aggregate and the DTO so the copy reads 'you tend to feel
better before your resting heart rate settles' instead of a context-free
gap figure.

The aggregate tallies which vital produced a real return across the
episodes that qualified for the typical-gap median and reports the most
frequent one (deterministic alphabetical tiebreak); the driver is named
only when a gap actually clears the magnitude gate. Zero new compute — the
returns were already in hand. Adds the localized copy in all six locales.

docs/api/openapi.yaml

@@ -20434,6 +20434,10 @@ components:
             type: integer
             minimum: -9007199254740991
             maximum: 9007199254740991
+        gapDriverType:
+          anyOf:
+            - type: string
+            - type: "null"
       required:
         - windowDays
         - episodeCount
@@ -20442,10 +20446,12 @@ components:
         - gapSampleSize
         - byMonth
         - byType
+        - gapDriverType
       additionalProperties: false
       description: "Cross-episode retrospective summary over the trailing window: episode + resolved counts, the typical
-        (median) recovery gap (null below the min-sample floor — withholds a thin claim), a recurrence-by-month tally,
-        and a per-type breakdown. Retrospective only."
+        (median) recovery gap (null below the min-sample floor — withholds a thin claim), a recurrence-by-month tally, a
+        per-type breakdown, and the dominant driving vital (`gapDriverType`, null when no gap is surfaced).
+        Retrospective only."
     DisclaimerAckEnvelope:
       type: object
       properties: