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.

proto/worldmonitor/resilience/v1/get_resilience_score.proto

@@ -14,8 +14,9 @@ message GetResilienceScoreResponse {
   double overall_score = 2;
   string level = 3;
   repeated ResilienceDomain domains = 4;
-  double cronbach_alpha = 5;
+  reserved 5;
   string trend = 6;
   double change_30d = 7;
   bool low_confidence = 8;
+  double imputation_share = 9;
 }

proto/worldmonitor/resilience/v1/resilience.proto

@@ -6,6 +6,8 @@ message ResilienceDimension {
   string id = 1;
   double score = 2;
   double coverage = 3;
+  double observed_weight = 4;
+  double imputed_weight = 5;
 }
 
 message ResilienceDomain {