perf(table): search global index fields concurrently

[QuakeWang]

Purpose

The global index scanner only parallelized shard reads within a single field. For multi-field AND predicates, indexed fields were still evaluated sequentially, leaving the configured I/O concurrency underutilized.

Brief change log

• Evaluate independent indexed fields concurrently.
• Share one scan-scoped semaphore across all shard reads so total I/O concurrency stays within global-index.thread-num.
• Add regression coverage for result correctness and the shared concurrency limit.

Tests

• cargo fmt --all -- --check
• cargo clippy --locked --all-targets --workspace --features fulltext,vortex -- -D warnings
• cargo test --locked -p paimon --all-targets --features fulltext,vortex

API and Format

Documentation

[JingsongLi]

+1