Add million-file metadata verification

[forhappy]

Summary

• add a metadata-only Prolly verifier for 1M logical files with 20-byte payload sizes
• cover both a raw key/value tree and repository-shaped current-object/version trees
• keep the durability lane out of the measurement and assert zero object-plane/payload traffic
• avoid re-encoding the logical version body during ObjectVersion ID derivation
• add a regression assertion that the optimized derivation preserves the public ID

Why

The verification isolates metadata performance from provider and payload durability latency. It demonstrates that grouped Prolly mutations are high-throughput, while repository metadata construction and canonical encoding dominate the state-shaped workload.

Validation

• cargo test -p silo-s3-core
• cargo test -p silo-s3-core --test immutable_payload
• cargo check --workspace
• cargo fmt --all -- --check
• git diff --check
• release metadata runs at 1M files with zero object-plane requests