feat: implement ReplacePartitions (two-PR plan)

[shangxinli]

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