logical,txn: replan LDR at source schema-change boundaries

When the destination reaches a source schema-change boundary, finish the current flow and rebuild the replication plan using the new source schema. The boundary must be reached across all source partitions before the old flow ends.

Some existing progress tracking, metadata forwarding, and clean-shutdown behavior will need adjustment to make this reliable.

**Out of scope:** Detecting the boundary on the source or enabling `ADD COLUMN`.

**Done when:** LDR reaches the boundary, records the correct replicated time, and replans using the post-change schema. Replication continues without losing the first non-NULL value written under the new schema.

**Notes / risk:** Quiet partitions and restarts during boundary handling need coverage.

**Depends on:** #173603, #173604

Jira issue: CRDB-66918

Epic CRDB-66920

No files, tests, or concrete entry points are named. Start by tracing logical transaction LDR handling for source schema-change boundaries, including progress tracking, metadata forwarding, and clean shutdown. Add coverage for quiet partitions and restarts; done means the boundary is reached across all partitions, the replicated time is correct, replanning uses the new schema, and the first non-NULL post-change value is preserved.

Written by the indexing model from the issue text.