Support secondary compression for RLE child buffers

[Xuanwo]

Summary

Allow RLE child buffers, especially values and run_lengths, to use additional compressive encodings such as bitpacking or general compression where compatible.

Scope

• Define which child-buffer compressions are legal for RLE values and run lengths.
• Preserve random-access and miniblock decoding constraints.
• Avoid changing existing GeneralMiniBlockCompressor semantics in a way that breaks old data.
• Add explicit validation so unsupported nested compression combinations fail clearly.

Acceptance Criteria

• RLE child buffers can use supported secondary compression.
• Existing RLE data remains readable.
• Unsupported nested compression combinations are rejected with clear errors.
• Tests cover compressed values, compressed run lengths, and invalid combinations.