refactor(resilience): remove Cronbach alpha, add imputationShare confidence (#2787)

* refactor(resilience): remove Cronbach alpha, add imputationShare confidence

Remove cronbach_alpha from proto (field 5 reserved) and all response
builders. Replace with imputationShare (field 9): the fraction of
weighted score from imputed (not observed) data.

lowConfidence now triggers on averageCoverage < 0.55 or
imputationShare > 0.40, replacing the unstable Cronbach-based gate.

weightedBlend() return type extended with observedWeight/imputedWeight
for provenance tracking through the scoring pipeline.

* fix(resilience): version cache key + fix IMF proxy imputation classification

1. Bump resilience score cache key to v2 to avoid serving stale
   cached responses missing imputationShare after deploy.
2. Add explicit `imputed` flag to WeightedMetric so proxy data
   (real IMF inflation with lower certaintyCoverage) is classified
   as observed, not imputed. Only synthetic absence-based scores
   count toward imputationShare.

docs/api/ResilienceService.openapi.yaml

@@ -111,9 +111,6 @@ components:
                     type: array
                     items:
                         $ref: '#/components/schemas/ResilienceDomain'
-                cronbachAlpha:
-                    type: number
-                    format: double
                 trend:
                     type: string
                 change30d:
@@ -121,6 +118,9 @@ components:
                     format: double
                 lowConfidence:
                     type: boolean
+                imputationShare:
+                    type: number
+                    format: double
         ResilienceDomain:
             type: object
             properties:
@@ -147,6 +147,12 @@ components:
                 coverage:
                     type: number
                     format: double
+                observedWeight:
+                    type: number
+                    format: double
+                imputedWeight:
+                    type: number
+                    format: double
         GetResilienceRankingRequest:
             type: object
         GetResilienceRankingResponse: