docs(format): define generic mini-block offsets

[Xuanwo]

Stack 9 of 10 for Lance generic block sequence compression.

This defines the Lance 2.3 generic mini-block offset grammar, codec descriptors, payload framing, strict-smaller selection rule, version gates, and random-read behavior in the format reference.

It also records that Lance 2.0–2.2 and SparseLayout retain their existing wire formats.

Stack navigation

• Position: 9 of 10
• Root: refactor(encoding): align block codecs with miniblock #8324
• Previous: feat(encoding): compress v2.3 mini-block offsets #8331
• Next: perf(encoding): measure compressed mini-block offsets #8333
• Top: perf(encoding): measure compressed mini-block offsets #8333

[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.