perf(vindex): split build timing logs by phase

[jerry-024]

Purpose

Vector index build diagnostics currently do not distinguish storage I/O, Parquet decoding, index training, and index population costs. This change adds opt-in phase-level timing logs so build bottlenecks can be identified without adding log noise when diagnostics are not enabled.

Enable the logs with:

PAIMON_LOG_VECTOR_INDEX_BUILD_TIMING=1

Brief change log

• Split source batch wait, OSS/file reads, and Parquet decoding into independent timings.
• Split raw temporary-file writes/rereads, index training, and index add into independent timings.
• Report serialization/upload, snapshot commit, total, unattributed time, and build counters in one structured log event.
• Keep timing collection and logging disabled by default.

Tests

• cargo test -p paimon
• Run vector index build tests with timing disabled and enabled; verify logs are emitted only when enabled and phase fields are populated.
• cargo fmt --all -- --check
• git diff --check

API and Format

No public API or storage format changes.

Documentation

No documentation changes. The environment variable is intended for opt-in build diagnostics.