refactor(vindex): trim granule planning overhead

[jerry-024]

Summary

Follow-up cleanup for #800:

• Return before byte-range aggregation when planning has already selected whole-shard fallback.
• Remove the synthetic aggregate byte range from the fallback granule; that range was never consumed.
• Remove unused derives and a no-op test clone, and tie the blocking-pool regression setup to QUEUE_CAPACITY.

This does not change the selected Granule plan or index output. It only skips work on paths that already return the whole-shard plan.

Validation

• 18 Granule pipeline unit tests
• 2 Parquet granule tests
• cargo +1.94.0 clippy -p paimon --lib --tests -- -D warnings
• git diff --check

[JingsongLi]

+1