feat: migrate multimapper-tiebreak RNG to in-tree splitmix64

[BenjaminDEMAILLE]

Summary

• Replace the rand crate (StdRng) with STAR-rs's splitmix64-based deterministic generator for the two RNG consumers: shuffle_tied_prefix (SE/PE --outMultimapperOrder Random tie-break) and pick_primary_and_mapq (transcriptome-space primary pick).
• Per docs-old/dev/porting-from-star-rs.md's RNG decision: this must land before STARsolo, whose UMI dedup depends on deterministic read order.
• Keeps the existing per-read seed (run_rng_seed + read-name hash, already thread-invariant) — only the underlying generator changes, matching STAR-rs's own choice of an in-tree splitmix64 over a general-purpose PRNG crate (STAR-rs's DIVERGENCES.md D16: STAR's own mt19937 output is itself thread-count-dependent, so no external crate's output is any more "correct" to match).
• Also migrates the test-only packed_stream.rs fuzz generator off rand, so the dependency drops from Cargo.toml entirely.

Test plan

• cargo build --all-targets / test / clippy -D warnings / fmt --check all green (455 tests)

[Psy-Fer]

This is worth adopting early, as it should clean up the comparisons/tests quite a lot.

We can later assess if we continue to use this method at a later date.

[Psy-Fer]

If you keep all your PRs on the same base commit, I can take care of merging them all. So no need to update each one.

[BenjaminDEMAILLE]

If you keep all your PRs on the same base commit, I can take care of merging them all. So no need to update each one.

Oh fine so. will let you do the merge :)