Avoid key clones in storage batch reads

[bjmeetsfo]

Summary

• coalesce storage batch reads over borrowed key references
• route multi-SSD batch reads without cloning temporary String keys
• preserve public get_batch behavior while trimming hot-path allocation

Validation

• cargo fmt -- --check
• cargo test --locked --no-default-features batch_get -- --nocapture
• cargo test --locked --no-default-features -q
• git diff --check HEAD~1..HEAD