feat: Phase U U5 evidence completeness index (ECI-) -- batch-level completeness snapshot, closes Phase U - #995

Summary

• Adds ECI- (Evidence Completeness Index) schema: top-level index aggregating PCC/CBA2/BEG/SAT across all batches
• AGGREGATED_SCHEMA_TYPES (4): PCC, CBA2, BEG, SAT
• VALID_ECI_GRADES: A (100%), B (>=75%), C (>=50%), D (<50%)
• Per-batch BatchSchemaPresence: presence flags for each of the 4 schema types, per-batch grade
• build() auto-computes overall fraction and grade from all batch entries
• dry_lab_only=True enforced; ECI- prefix validated
• 47 tests; BASELINE 10080→10143; closes Phase U (U1-U5 complete)

Test plan

• 47 unit tests pass: constants, build happy paths, validate rejections, format output
• Regression test passes
• Smoke test: B1 full + B2 partial + B3 empty → grade C (50%), B1 alone → grade A (100%)