Support rewrite_manifests table maintenance

### Feature Request / Improvement

Rewriting manifests without changing data speeds up scan planning when a table has accumulated many small manifests. Java exposes this as `Table.rewriteManifests()` (Spark: `rewrite_manifests`).

Scope: an explicit maintenance operation that reads the current snapshot's manifests, regroups entries, rewrites them preserving sequence numbers (entries rewritten as EXISTING) and, for V3, `first_row_id`, and commits a `replace` snapshot. Related: #270 tracks automatic manifest merging on write; this covers the explicit maintenance API.

定位与 Table.rewriteManifests() 对应的 Python 表维护/API 入口点；先阅读当前的快照和清单管理路径。完成的标准是：有一个显式操作对当前清单重新分组，保留序列号和 V3 first_row_id，将重写后的条目标记为 EXISTING，并提交一个替换快照。

由索引模型根据 Issue 内容生成。