feat: compress RLE child buffers

[Xuanwo]

Fixes #7329.

RLE miniblock compression currently stores run values and run lengths as raw Flat child buffers, so repeated or low-entropy child buffers can dominate the encoded page even after RLE chooses a better shape than raw fixed-width data.

This change lets RLE child buffers choose the smallest valid representation among Flat, OutOfLineBitpacking(Flat), and General(Flat) when a general compression scheme is configured. The decoder validates the supported nested child encodings and preserves the existing flat path, while rejecting combinations where both children require the run count.

The strategy automatically enables RLE child bitpacking and reuses explicit field compression settings for LZ4/Zstd child candidates, so configured general compression can reduce the values and run-length payload without adding a new public compression knob.

Performance data from a local release microbenchmark is below. Payload bytes exclude metadata; shape is values/run_lengths.

Scenario	Plain RLE	Auto child bitpack	LZ4 configured	Zstd configured
Long constant runs	384 B	384 B (flat/flat)	384 B (flat/flat)	384 B (flat/flat)
Short runs, small values	327,680 B	90,112 B (bitpack/flat)	5,120 B (general/general)	5,440 B (general/general)
Short runs, monotonic values	327,680 B	196,608 B (bitpack/flat)	132,224 B (bitpack/general)	132,800 B (bitpack/general)
Short runs, random values	327,680 B	286,720 B (flat/bitpack)	263,296 B (flat/general)	263,872 B (flat/general)
Variable U16 run lengths	26,226 B	26,226 B (flat/flat)	26,226 B (flat/flat)	26,226 B (flat/flat)

The encoder falls back to Flat when a child encoding does not shrink the payload. In the high-benefit cases, child bitpacking keeps encode/decode time in the same order of magnitude as plain RLE, while LZ4/Zstd trade more CPU for substantially smaller payloads. For example, short runs with small values measured about 512/237 us encode/decode for plain RLE, 517/248 us with child bitpacking, 585/265 us with LZ4, and 633/294 us with Zstd.

[codecov]

Codecov Report

❌ Patch coverage is 77.68969% with 197 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance-encoding/src/encodings/physical/rle.rs	82.17%	92 Missing and 36 partials ⚠️
rust/lance-encoding/src/compression.rs	58.18%	51 Missing and 18 partials ⚠️

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