feat(index): implement stable partition mapping readers

[LuQQiu]

What this adds

A stable-partition rewrite reads source fragments in scan order and distributes their live rows across an ordered list of destination fragments. Row order is preserved within each destination, but the destination cannot be inferred from source row order as it can for ordered compaction.

This PR implements the mapping as one immutable Lance file and exposes it through the MappingReader interface introduced in #9106. It does not integrate dataset lineage traversal or change the legacy compaction reader/writer.

Encoding

The row-map file contains one entry for every physical source row, ordered by source fragment and then row offset:

stable_partition.lance
├── label: nullable UInt16
│   ├── 0..N-1 → position in the ordered destination list
│   └── null   → source row was deleted
└── global buffer: cumulative counts matrix

For example, a label of 2 selects destinations[2]; it is not a fragment ID or destination row offset.

The writer receives labels for live rows in source scan order and inserts nulls from the source deletion vectors. This preserves one file row per physical source row, including rows already deleted when the rewrite ran.

The counts matrix divides the label column into 65,536-row blocks. Its on-disk layout is:

28-byte header
├── magic:                "LSPC"
├── version:              u32
├── representation:       u32 (0 = dense)
├── num_destinations:     u32
├── rows_per_block:       u32
└── total_source_rows:    u64

dense grid
└── cumulative u32 count for each (block, destination), block-major

The Lance schema metadata key lance:stable_partition:counts_buffer_index identifies this global buffer.

Address translation

For source row (fragment_id, row_offset), the reader first converts it to its position in the concatenated source layout. A null label returns a deleted row. Otherwise:

destination fragment = destinations[label]
destination offset   = count(label before this block)
                     + rank(label earlier in this block)

Because each destination is written in source scan order, this reconstructs its physical destination offset without storing a complete source-to-destination address table. Reordering rows within a destination is outside this encoding and requires a different mapping type.

Opening the mapping reads and validates the counts metadata without reading labels. Point and batch translations lazily read only the touched label blocks; batch requests sweep each touched block once.

Validation

The reader validates the label schema, counts header and dimensions, monotonic cumulative counts, source and destination row totals, label bounds, and per-block label/count agreement.

Validated with the stable-partition row-map and mapping-reader tests, workspace Clippy, and cargo fmt --all.

[lance-gatekeeper]

✅ Gate recommendation: approve.

The label-plus-cumulative-count encoding fits the stable-partition contract: it preserves per-destination input order without an explicit destination/offset entry per row. The immutable writer/reader and lazy MappingReader implementation keep validation and block traversal within this layer. This stacks cleanly after #9106 supplies the reader contract; downstream integration remains isolated to #9067, #9068, and #9107.

[lance-gatekeeper]

✅ Gate recommendation: approve.

Labels plus cumulative counts remain the right fit for the stable-partition contract: they preserve per-destination input order without an explicit destination/offset entry for every row. #9136 now establishes the unified history format; this PR is safely sequenced after #9106, while decoding, planner, and consumer integration stays isolated to #9067, #9068, and #9107.