Read generic variable-width offsets

[Xuanwo]

Goal

Teach the Lance 2.3 variable-width reader to consume generic block-compressed offsets before any writer emits the new layout.

Scope

• Decode offsets using descriptor-derived payload arity.
• Validate cardinality, monotonicity, zero base, terminal offset, buffer counts, and arithmetic bounds.
• Cover scan, range, take, multi-chunk, and malformed inputs.
• Preserve legacy interleaved-flat reads.

Context

This is PR 4 of the Generic Block Compression stack extracted from #8002. It must land before the writer change.