Avoid hash coalescing for small storage batches

[bjmeetsfo]

Summary

• add small-batch storage key coalescing without HashMap allocation
• use the helper for SSD and multi-SSD batch reads
• preserve duplicate-key output ordering

Validation

• cargo fmt -- --check
• cargo check --locked --no-default-features -q
• cargo test --locked --no-default-features -q storage_engine -- --test-threads=1
• cargo test --locked --no-default-features -q multi_ssd -- --test-threads=1