feat: add opt-in vector search timing diagnostics

[jerry-024]

Purpose

Add opt-in diagnostics for identifying latency and I/O amplification in batch vector search. Logging is disabled by default and enabled only when PAIMON_LOG_VECTOR_SEARCH_TIMING=1.

Brief change log

• Report API and evaluation timings for snapshot/manifest loading, global-index search, refinement, raw fallback, result merging, and finalization.
• Report vindex reader lifecycle timings, native batch chunk sizes, scalar fallbacks, concurrency, and memory budget.
• Track shared logical and physical range-read statistics across cloned vindex readers.
• Add ignored, controlled Parquet diagnostics for clustered, scattered, and missing-offset-index refinement reads.

Tests

• cargo fmt --all -- --check
• git diff --check
• cargo check -p paimon --lib (blocked on macOS ARM by paimon-vindex-core 0.3.0 using the unstable stdarch_neon_f16 feature)
• Run the ignored Parquet I/O diagnostics once the dependency build is available.

API and Format

No public API or storage-format changes. The new stderr diagnostics require the environment variable above.

Documentation

No documentation changes.