feat: support fixed-bucket partial-update merge engine

[QuakeWang]

Purpose

Linked issue: part of #256

This PR delivers a minimum usable merge-engine=partial-update implementation for primary-key tables with fixed buckets.
It shifts the earlier groundwork-heavy approach toward end-to-end functionality, while keeping only the validation that is still necessary for correctness.

This PR does not close #256 yet. Support for bucket=-1 partial-update, including cross-partition existing-key behavior, remains follow-up work.

Brief change log

• support fixed-bucket merge-engine=partial-update end to end for primary-key tables
• keep fail-fast for unsupported advanced partial-update options
• remove redundant validation and generic guard rails
• implement the read/write path changes required by basic latest-non-null partial-update semantics
• keep bucket=-1 and deletion-vectors.enabled=true explicitly unsupported in this PR
• add fixed-bucket E2E and targeted regression tests

Tests

API and Format

Documentation

[JingsongLi]

Can we simplify some of the code? There's no need for so much validation; we just need to focus on E2E and functionality.