feat: drop rand for in-tree splitmix64 (src/rng.rs). Empirically

[Psy-Fer]

Drop the rand dependency for an in-tree splitmix64 RNG (completes #118)

Replaces the external rand crate with a small in-tree deterministic RNG (src/rng.rs, splitmix64). This is #118's original goal — done correctly this time, and only after empirically proving the RNG choice is output-neutral

Why

rustar-aligner's only randomness is tie-breaking and EmptyDrops Monte-Carlo — paths where the exact RNG stream is not STAR-faithful anyway (STAR uses mt19937; we already diverge and report a tie-adjusted metric). What actually matters is determinism (reproducible regardless of thread scheduling), which a fixed-seed splitmix64 gives without pulling in rand → rand_core → rand_chacha → ppv-lite86 → zerocopy. Fewer deps = simpler build and a step toward the pure-Rust / C-toolchain-free goal.

#118 (BenjaminDEMAILLE) proposed exactly this but just deleted rand from Cargo.toml without migrating the WeightedIndex users, so it broke the build. This PR does the migration.

Empirical validation (the key point)

Before touching anything, I measured whether the RNG choice changes results, on the mouse 10x set (test/data/singlecell/, --soloCellFilter EmptyDrops_CR) — the one path claimed to "need rand":

• StdRng vs splitmix64: identical 229 cells (0 differ). The RNG choice makes no difference to the EmptyDrops calls.
• Both hit 228/229 vs STAR — same single borderline FDR cell (Monte-Carlo noise), so splitmix64 is exactly as STAR-faithful as StdRng.
• Deterministic across thread counts (4-thread ≡ 1-thread).

So WeightedIndex is just a cumulative-weight sampler (~15 lines), and removing rand costs nothing in faithfulness.

Changes

• src/rng.rs (new): SplitMix64 (next_u64/uniform/below), shuffle_deterministic (Fisher–Yates), cumulative_weights + sample_cumulative (WeightedIndex-equivalent). Unit-tested (determinism, range, permutation, weighted distribution).
• Migrated all four production rand sites to it:
  • solo/count.rs — EmptyDrops ambient Monte-Carlo sampler.
  • src/bin/emptydrops.rs — standalone binary, same sampler.
  • align/read_align.rs — --outMultimapperOrder Random tie-break shuffle.
  • lib.rs::pick_primary_and_mapq — --runRNGseed primary tie-break.
  • (+ one #[cfg(test)] in index/packed_stream.rs.)
• Dropped rand from Cargo.toml; rand/rand_core/rand_chacha/ppv-lite86/zerocopy are gone from the dependency tree.

Validation

• EmptyDrops: 229 cells, byte-identical to the pre-change StdRng output; 228/229 vs STAR; deterministic across thread counts.
• Yeast benchmark unchanged: SE 8788/8926, PE 8390 both-mapped (the tie-break stream change is output-neutral — tie-adjusted faithfulness holds).
• 579 tests pass (+5 new rng unit tests) · clippy --all-targets 0 warnings · fmt clean.