Track physical column layout optimization

Goal

Add an explicit physical-only maintenance operation that groups existing top-level fields into fewer standard per-fragment data files without changing logical rows or dataset semantics.

Context

Implementation and benchmark: https://github.com/lance-format/lance/pull/8614

Contract

• Callers choose top-level field groups based on their workload.
• Rewrites preserve logical values, fragment IDs, physical row order, deletion files, stable row IDs, and row-version metadata.
• A fixed snapshot cutoff materializes visible overlays while newer overlays remain authoritative.
• Disjoint field-level work can rebase safely; overlapping output conflicts fail.
• Index coverage for rewritten fields is invalidated conservatively.
• Legacy Blob, Blob v2, nested Blob v2, and external references use the existing safe rewrite machinery.
• The operation remains explicit policy and does not add an automatic scheduler.

Out of scope

• Automatic field grouping or workload learning.
• Changing logical row layout or row addresses.
• Immediate deletion of superseded provider objects.
• Automatic index rebuild scheduling.

Completion gates

• https://github.com/lance-format/lance/pull/8614 is rebased, passes CI, is reviewed, and is merged.
• Rewrite, overlay, conflict, index, Blob, and stable-row-ID coverage passes.
• Representative Local FS and object-storage benchmarks retain exact logical checksums.
• Read-amplification and rewrite-cost trade-offs are documented.