feat(file_index): add Java-compatible Bitmap V2 writer

[QuakeWang]

Purpose

Related issue: #34

The Bitmap file-index module was read-only. It could consume Java V1/V2 payloads but could not produce the V2 format used by Java Paimon 2.0, leaving Rust without a compatible Bitmap write path.

Brief change log

• Add a crate-private Java-compatible Bitmap V2 writer.
• Write row/null and per-value RoaringBitmap32 data.
• Support singleton offsets, V2 metadata, secondary index blocks, and index-block-size.
• Preserve Java-compatible value encoding.
• Reject invalid configuration and unsupported data types.
• Add Java golden and Rust reader tests for nulls, singletons, multiple bitmap bodies, multiple blocks, and variable-length block boundaries.

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
• Java release-2.0 golden and reader interoperability probes

API and Format

No public API change. This adds Java-compatible Bitmap V2 payload writing. V1 writing remains unsupported.

Documentation

No documentation changes.