feat(encoding): encode generic mini-block offsets

[Xuanwo]

Stack 7 of the Generic Block Sequence Compression series.

Depends on #8044.

This layer adds the generic variable-width mini-block writer capability. It computes complete serialized wire cost before selecting a layout, fixes one reusable concrete offset compressor for the page, and writes metadata-only or one-payload offset codecs without changing chunk-local read granularity. It also promotes the concrete block compressors needed by the production writer out of test-only compilation.

The default compression strategy does not select this path yet, so persisted writer behavior remains unchanged in this layer. The next stack layer is the Lance 2.3 adopter and version gate.

Validation:

• cargo check -p lance-encoding --no-default-features --tests
• cargo check -p lance-encoding --tests --benches
• cargo test -p lance-encoding encodings::physical::binary::tests
• cargo test -p lance-encoding encodings::logical::primitive::miniblock::tests
• cargo clippy --all --tests --benches -- -D warnings
• cargo fmt --all

Stack navigation

• Umbrella / integration reference: feat(encoding): add generic block compression #8002
• Previous: feat(encoding): select bounded offset codecs #8044
• Next: feat(encoding): adopt generic offsets in Lance 2.3 #8046