Add an optimized ASCII write path in ByteBufferBsonOutput.writeCharacters for Netty buffers. Today, Netty (non-array-backed) buffers fall back to writeOnBuffers, which performs character-by-character relative put(byte) with inline UTF-8 encoding and buffer boundary checks in one complex loop.

This optimization adds a dedicated ASCII fast-path for Netty buffers (tight loop, minimal branching, JIT-friendly).

Evidence

Netty transport insert benchmarks vs main show improvements with no regressions:

• Large doc bulk insert: +14.89%
• Large doc insertOne: +13.99%
• Large doc Client BulkWrite insert: +10.59%
• Small doc Collection BulkWrite insert: +5.83%
• Small doc bulk insert: +5.66%
• Small doc Client BulkWrite insert: +4.74%

5/8 insert benchmarks show statistically significant improvement (z > 2). No regressions.

Proposed Changes

• Detect Netty-backed buffers in writeCharacters.
• For ASCII, write bytes via ByteBuf.setByte(bufferPos + i, c) in a tight loop.
• Fall back to existing path for non-ASCII.

Acceptance Criteria

• ASCII path for Netty ByteBuff - similarly to what has been done for JDK's ByteBuffer.
• Non-ASCII behavior unchanged and correct.
• Existing tests pass.
• Netty insert benchmarks show the expected improvement trend with no regressions.