OnPair trainer should draw Rust's sample (StdRng + partial_shuffle) for byte-identical dictionaries #534

Problem

OnPairEncodingEncoder does not draw the same training sample as the Rust onpair crate, so it trains different dictionaries on identical data. Prime directive: behavioral parity with Rust.

Found by comparing a 100k-row URL column (https://example.com/products/<n>/reviews?page=<m>, 7 chunks of 16,384 rows) written by vortex-java and by Rust 0.88.0 (the release vortex-jni uses), both at cascade 3 with global dict off. Both pick vortex.onpair per chunk.

dict_bytes_len per chunk:

Chunks 0-4 end up within 0.04% in total, but Java's chunk 5 is 18% smaller (131,904 B vs 161,507 B): its trainer built a 761-token dictionary and the cascade then dict-encoded the 467 distinct codes. That one chunk is 29.6 KB of the 31.6 KB (3.2%) by which Java's file is smaller. This is training luck, not a systematic advantage, and it could go the other way on other data.

Cause

• Rust (onpair 0.2.1, training_order in encoding/trainer.rs): StdRng::seed_from_u64(42) (ChaCha12, rand 0.9.5) and order.partial_shuffle(&mut rng, k) with k = min(n, 2 * sample_fraction * n + 1024). Training consumes only the selected tail order[n-k..]; if that tail is shorter than the byte budget, k doubles and the shuffle restarts.
• Java (OnPairEncodingEncoder.Encoder.train): SplittableRandom(42), a full Fisher-Yates shuffle, and training consumes order from the start.

Proposed fix

1. Port rand 0.9.5 partial_shuffle on top of SampleRng (already a bit-exact ChaCha12 StdRng and random_range for u32). It uses IncreasingUniform / calculate_bound_u32, which batches several swaps into one random chunk. Check that SampleRng.nextIntInclusive (written against rand 0.10) gives the same values as 0.9.5's random_range.
2. Reproduce the k computation, the tail consumption and the doubling retry loop in train.
3. Parity test: the URL column above, Rust as the oracle, comparing dict_bytes_len and encoded size per chunk (expect identical).

Repro

Scratch crate with vortex = "=0.88.0"; data dumped from the Java CascadeDepthGainIntegrationTest patterns (utf8-urls), seed 42. Not committed.