feat: add Dataset.sample() with optional seed for deterministic sampling

[beinan]

Summary

• Add Dataset.sample() API to both Rust core and Java JNI for random row sampling
• Add optional seed parameter for deterministic sampling — same seed + same dataset state = same result
• Useful for reproducible ML training/validation splits

Changes

• rust/lance/src/dataset.rs: Add seed: Option<u64> param to Dataset::sample(); use StdRng::seed_from_u64 when seeded, rand::rng() otherwise
• rust/lance/src/dataset/tests/dataset_io.rs: Add test_sample_deterministic_with_seed verifying same seed = same result, different seed = different result
• rust/lance/src/index/vector/utils.rs, rust/lance/src/dataset/write/merge_insert.rs, rust/lance/benches/take.rs: Pass None for existing callers (backward compatible)
• java/lance-jni/src/blocking_dataset.rs: Add nativeSample JNI function
• java/src/main/java/org/lance/Dataset.java: Add sample(n, columns), sample(n, columns, fragmentIds), and sample(n, columns, fragmentIds, seed) overloads
• java/src/test/java/org/lance/DatasetTest.java: Test basic sampling, fragment-filtered sampling, over-sampling, and deterministic sampling with seed

Test plan

• Rust: test_sample_deterministic_with_seed — same seed = identical RecordBatch, different seed = different result
• Rust: existing test_sample_with_fragment_ids, test_sample_with_empty_fragment_ids_rejected, test_sample_with_unknown_fragment_ids_rejected pass with None seed
• Java: testSample — basic sample, fragment-filtered sample, over-sample, deterministic seed verification

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/dataset.rs	87.50%	1 Missing ⚠️
rust/lance/src/index/vector/utils.rs	50.00%	1 Missing ⚠️

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