feat(update): add OverwriteFiles for overwrite snapshot commits

[lishuxu]

Summary:
Add a production OverwriteFiles builder that brings iceberg-cpp to semantic parity with Java's BaseOverwriteFiles. It supports explicit file replacement (DeleteFile + AddFile) and range-based replacement (OverwriteByRowFilter + AddFile) with the same family of pre-commit concurrency validations. The builder is a thin subclass of MergingSnapshotUpdate and reuses the existing commit kernel (Apply/summary/retry/cleanup) unchanged.

Changes:

• New OverwriteFiles class (src/iceberg/update/overwrite_files.{h,cc}) and Table::NewOverwrite() / Transaction::NewOverwrite() entry points.
• Builder surface: AddFile, DeleteFile, bulk DeleteFiles, OverwriteByRowFilter, ValidateFromSnapshot, ConflictDetectionFilter, ValidateNoConflictingData, ValidateNoConflictingDeletes, ValidateAddedFilesMatchOverwriteFilter, WithCaseSensitivity.
• Validate(): conflict-filter resolution, concurrent add/delete conflict checks, and strict added-file range validation (projection + StrictMetricsEvaluator).
• Tests (overwrite_files_test.cc, 45 cases) and CMake/meson wiring.

Behavior alignment with Java:

• operation() returns append/delete/overwrite from builder content.
• Conflict-filter resolution mirrors BaseOverwriteFiles (explicit -> row filter -> AlwaysTrue); replaced-file delete checks honor ConflictDetectionFilter.
• Strict added-file validation uses a single DataSpec(), rejecting multi-spec and empty added-file sets.
• Deviations: public WithCaseSensitivity (vs caseSensitive) to avoid a protected-name clash; ValidateFromSnapshot rejects negative ids early.