feat(encoding): adopt generic offsets in Lance 2.3

[Xuanwo]

Stack 8 of the Generic Block Sequence Compression series.

Depends on #8045.

This is the first production adopter. Ordinary Lance 2.3 variable-width mini-block pages may select the generic offset container when its complete serialized wire size is strictly smaller. Lance 2.0 through 2.2 retain legacy interleaved Flat offsets byte-for-byte; explicit outer general compression retains legacy offsets; SparseLayout remains on its existing wire.

Page-level coverage verifies Range, Delta, Delta(Range), legacy fallback, unchanged chunk-local take request shape, and the SparseLayout exclusion. The format reference documents the two unambiguous mini-block containers and the bounded block-sequence contract.

Validation:

• cargo check -p lance-encoding --no-default-features --tests
• cargo check -p lance-encoding --tests --benches
• cargo test -p lance-encoding test_v2_3_
• cargo test -p lance-encoding test_variable_offset_codec_is_version_gated
• cargo clippy --all --tests --benches -- -D warnings
• cargo fmt --all
• make -C docs build

The docs build succeeded with existing missing-external-doc-repository and link warnings; its generated tracked-file side effects were restored before commit.

Stack navigation

• Umbrella / integration reference: feat(encoding): add generic block compression #8002
• Previous: feat(encoding): encode generic mini-block offsets #8045
• Next: perf(encoding): benchmark generic offsets #8048

[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.