fix(encoding): bound general block decompression

[Xuanwo]

Stack 6 of 10 for Lance generic block sequence compression.

General block decompression now validates descriptor structure, declared output sizes, buffer counts, and allocation bounds before dispatching to a concrete codec. This closes the remaining unbounded path needed by nested generic codec trees.

Valid persisted bytes and reader results are unchanged; malformed metadata now fails before allocation or codec execution.

Stack navigation

• Position: 6 of 10
• Root: refactor(encoding): align block codecs with miniblock #8324
• Previous: fix(encoding): validate block bitpacking payloads #8328
• Next: feat(encoding): select compressed offset blocks #8330
• Top: perf(encoding): measure compressed mini-block offsets #8333