Unify RewriteManifestsAction with SnapshotProducer once Replace/Delete ops are supported

### Problem

`RewriteManifestsAction` (added in #2543) intentionally does **not** plug into `SnapshotProducer`. The producer path has four cross-cutting assumptions that block a clean integration today:

1. **`update_snapshot_summaries` rejects `Operation::Replace`** — `crates/iceberg/src/spec/snapshot_summary.rs:337-346` returns `ErrorKind::DataInvalid` for any operation other than `Append` / `Overwrite` / `Delete`. `SnapshotProducer::commit()` calls `update_snapshot_summaries` unconditionally, so the producer cannot emit a `Replace` snapshot at all today.
2. **`ManifestProcess::process_manifests` is synchronous** — `crates/iceberg/src/transaction/snapshot.rs:119-125`. Rewrite-manifests must write *new* manifests (async I/O via `ManifestWriter`), then return them as the post-process output. There is no async seam.
3. **Summary keys are driven by `added_data_files` only** — `crates/iceberg/src/transaction/snapshot.rs:373-379`. For rewrite-manifests `added_data_files` is empty by definition, so no `total-*` carry-forward happens, and there is no hook for the rewrite-specific summary keys (`manifests-created`, `manifests-replaced`, `manifests-kept`, `entries-processed`).
4. **`manifest_file()` shape mismatch** — the producer's `manifest_file()` assumes "if `added_data_files` non-empty, write one manifest from those files." Rewrite-manifests produces an N-to-M rewrite of *existing* manifest entries; the shape is fundamentally different.

### Scope

Refactor once the underlying primitives land:
- Add `Operation::Replace` (and `Operation::Delete`) to `update_snapshot_summaries`' allow-list, with summary-key carry-forward semantics that mirror Java `BaseRewriteManifests`.
- Make `ManifestProcess::process_manifests` async (or add a parallel async hook) so producers can emit manifests written via `ManifestWriter`.
- Decouple summary-key emission from `added_data_files` so rewrite-style ops can declare their own summary additions.
- Migrate `RewriteManifestsAction` onto the unified path and delete the duplicated commit logic.

Out of scope for this issue: the `rewrite_if` predicate, `cluster_by`, custom `spec_id`, custom `staging_location`, and the `iceberg-datafusion` SQL-procedure layer — all separately deferrable from #2543.

### References

- Java reference: `apache/iceberg` `BaseRewriteManifests` + `SnapshotProducer`
- Proposal: #1453 (closed)
- Implementation: #2543

Start by reading crates/iceberg/src/spec/snapshot_summary.rs:337-346 and crates/iceberg/src/transaction/snapshot.rs:119-125, 373-379, then compare #2543 with the Java BaseRewriteManifests and SnapshotProducer references. Done means Replace/Delete summaries, async manifest processing, rewrite-specific summary additions, and manifest shape support are unified so RewriteManifestsAction no longer duplicates commit logic.

Written by the indexing model from the issue text.