Migrate pecos-random to rapidrand with stream-identical seeding

[ciaranra]

rapidhash 4.5 deprecated its rng module ("use the rapidrand crate instead"), which is what tripped PR #355's lint gates and led to the temporary <4.5 hold. This is the proper migration.

The determinism question, settled empirically

Seeded reproducibility is load-bearing (users record seeds to reproduce runs), so the swap was gated on stream identity, not assumed:

• rapidrand's mixing function and secrets are byte-identical to rapidhash 4.4's rapidrng_fast (verified against both crates' sources).
• The seeding paths differ: old RapidRng::new(seed) stored the seed raw; rapidrand's seed_from_u64 pre-mixes (different stream), while SeedableRng::from_seed(seed.to_le_bytes()) is raw. Verified with a side-by-side harness over seeds {0, 1, 42, 0xDEADBEEF, u64::MAX}: from_seed is bit-identical to the old stream; seed_from_u64 is not.

Changes

• New private pecos_random::rapid_rng::RapidRng shim wrapping rapidrand::RapidRng with the legacy semantics (new(u64) = raw from_seed; next()), documented in one place — all existing call sites in rng.rs/scalar_rng.rs unchanged.
• New golden test tests/seed_stream_stability.rs: locks the first 8 outputs of ParallelRapidRng and PecosScalarRng at seed 42 to values captured from the pre-migration implementation and verified passing before the swap — any future accidental reproducibility break fails loudly.
• statistical_quality.rs and the RNG benchmarks migrated the same way (raw from_seed, so bench comparability holds).
• rapidhash removed from the workspace entirely (no other consumers); rapidrand 0.1.2 added.

Verification

Golden stream test passes on both sides of the migration (the actual oracle); full pecos-random suite green; cargo check --workspace clean; clippy clean on pecos-random (all targets) + benchmarks; fmt clean.

Merge-order note: #355 holds rapidhash = "4, <4.5"; this PR deletes the dependency. Whichever merges second takes a trivial Cargo.toml/Cargo.lock conflict — happy to rebase this one after #355 lands.