feat: add RLE v2 run length widths

[Xuanwo]

Summary

Adds RLE v2 run-length widths so newly created datasets can write RLE pages with u16 or u32 run lengths instead of splitting every run at 255 values. The capability is recorded as a reader feature flag and is only enabled when a new dataset is created with WriteParams::enable_rle_v2; existing unflagged datasets reject attempts to turn it on mid-stream.

Closes #7327.

Benchmark

Ran on xuanwo-lance-lazy-metadata-bench with a #6941-style sorted low-cardinality asset_id workload.

workload	Lance default	Lance RLE2	reduction
150M rows / 5k assets / random5 value	167.36 MiB	164.57 MiB	1.67%
150M rows / 5k assets / by-asset5 value	7.62 MiB	2.03 MiB	73.34%

The first row keeps the random low-cardinality value column from the issue-like workload, which dominates total size. The second row isolates the long-run case RLE2 targets.

Validation

Validated with focused RLE2 tests and full Rust clippy before publishing.

[github-actions]

Important

This PR touches the Lance format specification.

Substantive changes to the format specification — the .proto definitions
and the spec docs under docs/src/format/ — require a PMC vote before merge.
Minor edits such as typo fixes, wording, or formatting are excluded; use your
judgment.

If this is a meaningful format change:

• Start a vote following the Lance community voting process.
  Format specification modifications need 3 binding +1 votes (excluding the
  proposer), held on GitHub Discussions, with a minimum voting period of 1 week.
• Once the vote passes, link the completed vote in this PR. It should not be
  merged until the vote is linked.

[codecov]

Codecov Report

❌ Patch coverage is 84.04558% with 112 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-encoding/src/encodings/physical/rle.rs	78.69%	67 Missing and 8 partials ⚠️
rust/lance-encoding/src/compression.rs	86.36%	9 Missing and 6 partials ⚠️
rust/lance/src/dataset/write/commit.rs	83.75%	12 Missing and 1 partial ⚠️
rust/lance/src/dataset/write.rs	92.22%	6 Missing and 1 partial ⚠️
rust/lance-namespace-impls/src/dir/manifest.rs	50.00%	0 Missing and 1 partial ⚠️
rust/lance/src/dataset/transaction.rs	93.75%	0 Missing and 1 partial ⚠️

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