feat: implement ReplacePartitions (two-PR plan)

## Overview

This issue tracks the implementation of `ReplacePartitions` in C++, assigned in #637.

`ReplacePartitions` performs dynamic partition overwrite: each file added to the operation marks its entire partition for deletion, and on commit, existing files in those partitions are atomically replaced. The resulting snapshot uses `operation="overwrite"` and summary property `"replace-partitions"="true"`.

## Implementation Plan

### PR 1 — Core implementation

New files:
- `src/iceberg/update/replace_partitions.h`
- `src/iceberg/update/replace_partitions.cc`

Modified files:
- `src/iceberg/snapshot.h` — add `kReplacePartitions` constant to `SnapshotSummaryFields`
- `src/iceberg/type_fwd.h` — add `class ReplacePartitions;`
- `src/iceberg/CMakeLists.txt` — register new source

### PR 2 — Table/Transaction API surface + tests

Modified files:
- `src/iceberg/table.h` / `table.cc` — add `NewReplacePartitions()`
- `src/iceberg/transaction.h` / `transaction.cc` — add `NewReplacePartitions()`

New files:
- `src/iceberg/test/replace_partitions_test.cc`
- `src/iceberg/test/CMakeLists.txt` — register test

## Design

`ReplacePartitions` extends `SnapshotUpdate` (same pattern as `FastAppend`) and uses `ManifestFilterManager` to mark existing partition files as DELETED:

- `AddFile(file)`: calls `filter_manager_.DropPartition(spec_id, partition)`, accumulates new file in `new_data_files_by_spec_`
- `ValidateAppendOnly()`: calls `filter_manager_.FailAnyDelete()`
- `Apply()`: for unpartitioned tables calls `filter_manager_.DeleteByRowFilter(AlwaysTrue)`; calls `FilterManifests()` on existing snapshot, then prepends new manifests
- `operation()`: returns `DataOperation::kOverwrite`
- `Summary()`: sets `"replace-partitions"="true"`

Relates to: #637

Beginne damit, die bestehenden Implementierungen von FastAppend und SnapshotUpdate zu lesen, und untersuche anschließend die aufgeführten Snapshot-, Manifest-, Tabellen- und Transaktionsdateien. Verwende die geplante replace_partitions_test.cc und deren CMake-Registrierung, um zu überprüfen, dass Partitionsersetzung, das Verhalten bei nicht partitionierten Tabellen, Operations- und Summary-Metadaten sowie die neuen Tabellen- und Transaktions-APIs abgedeckt sind.

Vom Indexierungsmodell aus dem Issue-Text verfasst.