fix(storage): prioritize semantic coverage queue chains

[fewensa]

Summary

• include semantic coverage-index-v2 queue size in compaction chain priority hints
• sort queued chains by semantic queue first, then total queue, while keeping rotating slots
• add regression coverage for semantic queue priority

Verification

• cargo fmt -p datalens-cli --check
• CARGO_TARGET_DIR=/code/helixbox/datalens/target CARGO_INCREMENTAL=0 cargo test -p datalens-cli storage_compaction_chain_batch -- --nocapture
• CARGO_TARGET_DIR=/code/helixbox/datalens/target CARGO_INCREMENTAL=0 cargo clippy -p datalens-cli --lib --tests -- -D warnings
• git diff --check