Core: Add branch support for RewriteManifests operation

[wombatu-kun]

What

Adds branch support to RewriteManifests via toBranch(String) so manifest compaction/clustering can target a named branch. Closes #15981.

RewriteManifests was the only SnapshotUpdate that could not commit to a branch: BaseRewriteManifests inherited the default SnapshotUpdate.toBranch, which throws UnsupportedOperationException. There was also a second, latent bug: even if toBranch were unblocked, apply(base, snapshot) read manifests from base.currentSnapshot() (always the main tip) rather than the snapshot argument that SnapshotProducer passes in (the tip of the target branch), so a branch commit would have rewritten the wrong manifests.

Changes

• Override toBranch to delegate to the inherited SnapshotProducer.targetBranch, which already rejects null and tag refs with IllegalArgumentException.
• Read the current manifests from the snapshot parameter (snapshot != null ? snapshot.allManifests(io) : Collections.emptyList()), mirroring FastAppend.apply and MergingSnapshotProducer.apply.

Rewriting an existing branch now compacts that branch's manifests without affecting main. Targeting a branch that does not exist yet forks it from main (parent = the current main snapshot), consistent with other SnapshotUpdate operations.

Tests

New tests in TestRewriteManifests, exercised across format versions v1-v4:

• testRewriteManifestsOnBranch: the branch diverges from main (an extra file appended only on the branch) before the rewrite, so it verifies the rewrite operates on the branch tip and leaves main untouched. This is the case that actually exercises the apply() fix.
• testRewriteManifestsCreatesBranchIfNeeded: rewriting to a non-existent branch forks it from main.
• testRewriteManifestsToBranchRejectsNullBranch and testRewriteManifestsToBranchRejectsTag: validation.

Notes

This revives the approach from the stale PR #15982 by @jlkvanloon (added as a commit co-author), with broader test coverage. In particular, the original happy-path test appended both files to main before branching, so main and the branch were identical at rewrite time and the test did not exercise the apply() fix.