Persist: Add new API for run based spine replacement

[DAlperin]

Motivation

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[Copilot]

Pull Request Overview

This PR adds a new API for run-based spine replacement in the persist client, extending the existing merge resolution functionality to support fine-grained replacement of specific runs within hollow batches rather than just entire batches.

• Adds RunLocation struct to identify specific runs within hollow batches using SpineId and RunId pairs
• Introduces a new apply_merge_res_checked method that can replace individual runs within batches
• Extends FueledMergeRes with inputs and new_active_compaction fields to support the new replacement logic

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/persist-client/src/internal/trace.rs	Core implementation of run-based spine replacement with new data structures and methods
src/persist-client/src/internal/state_serde.json	Test data updates reflecting the new run-based structure
src/persist-client/src/internal/state_diff.rs	Updates to populate new fields in FueledMergeRes
src/persist-client/src/internal/state.rs	Test utilities and method renaming for compatibility
src/persist-client/src/internal/machine.rs	Updates to construct FueledMergeRes with new fields
src/persist-client/src/internal/compact.rs	Updates to construct FueledMergeRes with new fields
src/persist-client/src/cli/admin.rs	Updates to construct FueledMergeRes with new fields

[bkirwi]

It's not really dead!