Add bounded generic-offset codec cascade

[Xuanwo]

Goal

Complete the bounded generic block compression cascade used by Lance 2.3 variable-width offsets.

Scope

• Add adopter-private statistics and selection.
• Compare Constant, Range, Delta, RLE, bitpacking, dictionary, General, and Flat where applicable.
• Add block dictionary and bounded nested codec combinations.
• Ensure only the winning candidate is fully materialized.
• Add a fixed winner corpus, safety tests, and representative benchmarks.

Context

This is PR 6 of the Generic Block Compression stack extracted from #8002. It must not introduce a parallel general-purpose planner framework.