feat(encoding): select compressed offset blocks

[Xuanwo]

Stack 7 of 10 for Lance generic block sequence compression.

This adds the variable-width offset statistics and selector that evaluate flat, range, delta, RLE, dictionary, and bitpacked representations by complete serialized cost. Selection is fallible, bounded to u32/u64 offsets, and retains flat data unless a candidate is strictly smaller.

The selector is not wired into a production file-version strategy in this layer, so persisted bytes remain unchanged.

Stack navigation

• Position: 7 of 10
• Root: refactor(encoding): align block codecs with miniblock #8324
• Previous: fix(encoding): bound general block decompression #8329
• Next: feat(encoding): compress v2.3 mini-block offsets #8331
• Top: perf(encoding): measure compressed mini-block offsets #8333