feat(encoding): add bounded block dictionary

[Xuanwo]

Stack 5 of the generic block compression series. Depends on #8042.

This PR adds the concrete block Dictionary codec without enabling production selection. Dictionary owns its u32 indices and typed item child codecs, exposes zero payload only when both children are metadata-only, and otherwise uses one bounded length-framed payload. Reader validation covers item count, child cardinality, frame boundaries, metadata-only child lengths, and index bounds.

Lance 2.0–2.2 selection and bytes remain unchanged. The encoder is exercised directly in tests and will be enabled by the following selector layer.

Validation:

• cargo fmt --all
• default and no-default cargo check for lance-encoding tests/benches
• cargo test -p lance-encoding compression::block
• cargo test -p lance-encoding compression::tests
• cargo test -p lance-encoding encodings::physical
• uv run mkdocs build from docs/
• cargo clippy --all --tests --benches -- -D warnings

Stack navigation

• Umbrella / integration reference: feat(encoding): add generic block compression #8002
• Previous: feat(encoding): decode generic mini-block offsets #8042
• Next: feat(encoding): select bounded offset codecs #8044

[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
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