feat(reject-history): add materialized Pareto aggregate layer with feature-flagged rollout
Pre-compute 6-dimension metric cubes from cached LOT-level DataFrames so
interactive Pareto requests read compact snapshots instead of re-scanning
detail rows on every filter change. Includes single-flight build guard,
TTL/size guardrails, cross-filter exclude-self evaluation, safe legacy
fallback, response metadata exposure, telemetry counters, and a 3-stage
rollout plan (telemetry-only → build-enabled → read-through).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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