feat(core): define mapping readers with ordered compaction support

[LuQQiu]

Stack reordered: the standalone format proposal is now #9136 (replacing #9065), followed by #9106 → #9064 → #9067 → #9068 → #9107 in native stack #9137. This PR’s diff is byte-identical to its pre-reorder diff. All branches include main 31d78d170; no history was rewritten. Formatting checks pass; Clippy and tests remain blocked by the unavailable object_store_opendal 0.60.1 dependency.

Define a MappingReader contract for single and batch physical-row-ID remapping (u64 to Option<u64>). Single-row remapping is required; batch remapping defaults to calling it in order, and implementations can override the batch path. The ordered-compaction adapter wraps the existing RowAddrRemap; it does not duplicate source/destination metadata. Coverage belongs to the common lineage reader. The legacy FRI reader and writer are unchanged.

This is the base of the six-PR FRI reader stack. Stable partitions implement the same contract in #9064; graph decoding remains in #9067, and graph composition/planner integration in #9068. Scalar and vector consumers follow together in #9107.

Validation: mapping contract tests cover default-batch delegation/error propagation, deleted rows, duplicates, out-of-order and empty batches, single/batch equivalence, invalid fragments, and offsets outside the source. Workspace Clippy (--all --tests --benches -- -D warnings) and cargo fmt --all passed.

No format documentation changes. Draft pending stack review.

Latest main synchronization (2026-09-10): merged d7b031f9c through the stack without rewriting history. cargo fmt --all and whitespace checks pass. Workspace Clippy could not run: main now requires object_store_opendal 0.60.1, but the crates.io index currently resolves only up to 0.60.0. Tests were not rerun after this synchronization because dependency resolution is blocked. Earlier validation above predates this merge.

[lance-gatekeeper]

✅ Gate recommendation: approve.

The mapping seam is appropriately scoped: ordered compaction keeps the existing bitmap/rank behavior, while the async contract lets external row-map implementations override batch reads for efficient I/O. The tests cover deletion, ordering, duplicate, invalid-address, and single/batch-equivalence semantics.

The current stack order is appropriate: land #9136 before this PR.