feat(encoding): decode generic mini-block offsets

[Xuanwo]

Stack 4 of the generic block compression series. Depends on #8041.

Variable-width mini-block decoding now distinguishes the existing interleaved Flat layout from a generic offset descriptor. The generic path consumes the descriptor-derived zero/one payload plus the values buffer and validates cardinality, zero origin, monotonicity, Arrow signed bounds, and the final value length. The legacy path keeps the existing wire shape while converting malformed buffer panics into contextual errors.

This is reader-only. Production writers still emit the legacy layout for every file version.

Validation:

• cargo fmt --all
• cargo check -p lance-encoding --no-default-features --tests
• cargo check -p lance-encoding --tests --benches
• focused generic Range/Delta and malformed legacy reader tests
• cargo test -p lance-encoding compression::tests
• cargo test -p lance-encoding encodings::physical
• cargo clippy --all --tests --benches -- -D warnings

Stack navigation

• Umbrella / integration reference: feat(encoding): add generic block compression #8002
• Previous: feat(encoding): add range and delta block codecs #8041
• Next: feat(encoding): add bounded block dictionary #8043