feat: allow tuning miniblock value chunks to 32k

[Xuanwo]

This allows miniblock writers to use up to 32K logical values per chunk when explicitly configured via LANCE_MINIBLOCK_MAX_VALUES, while keeping the default at 4096.

The file format already stores log_num_values in 4 bits, so the writer-side guard can allow values up to 15 without requiring the large-chunk metadata path. The compressed byte-size limits remain enforced.

Fixes #7326.

[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.

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[codecov]

Codecov Report

❌ Patch coverage is 71.69811% with 15 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
.../lance-encoding/src/encodings/logical/primitive.rs	67.39%	14 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!

[westonpace]

I think bumping the max up to 32Ki is fine. I think there is some small chance we might end up seeing the "too much rep/def information and it doesn't all fit in the miniblock" error again but we can tackle that problem a different way when it surfaces.