feat(encoding): add bounded block dictionary

[Xuanwo]

Stack 3 of 10 for Lance generic block sequence compression.

This adds a bounded block dictionary codec for u32/u64 sequences. Dictionary values and indices remain separate typed child codec trees, with explicit frame lengths, checked cardinality, and index-bound validation before materialization.

The codec is available for later composition but is not selected by a production writer in this layer. Stable Lance 2.0–2.2 bytes and reader behavior remain unchanged.

Stack navigation

• Position: 3 of 10
• Root: refactor(encoding): align block codecs with miniblock #8324
• Previous: feat(encoding): add range and delta block codecs #8325
• Next: feat(encoding): compose generic block codecs #8327
• 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.