fix(rayon): remove the aliasing unsafe from the parallel sweep

The parallel color-group sweep passed a `*mut SkillStore` through a `usize`
and cast it back inside the rayon closure, so every worker materialised its
own `&mut SkillStore` to the same store. Two live `&mut` to one object is an
aliasing violation whatever the workers subsequently touch — `&mut` carries
`noalias` down to LLVM — and laundering the pointer through `usize` also
discarded provenance. The existing SAFETY comment argued element
disjointness, which is true and is why nothing miscompiled in practice, but
it is not the property the aliasing rules ask about.

Events in a color group touch disjoint agents, so none can observe another's
writes. That makes the sweep separable rather than merely safe-in-practice:
`Event::compute` runs inference over shared `&self.skills` with no mutation,
and `Event::apply` folds the results in afterwards in index order. No
`unsafe`, no aliasing argument, and the apply order does not depend on which
worker finished first, so results stay bit-identical across thread counts.

The crate now contains no `unsafe` at all, locked in with
`#![forbid(unsafe_code)]`.

Splitting compute from apply also removes the duplicated sweep body: the
`from > 0` branch of `TimeSlice::iteration` was a verbatim copy of
`iteration_direct`, and both now share one implementation.

Cost, measured on the three `history_converge` workloads (sequential vs
parallel, this machine):

    500x100@10perslice     4.02ms -> 4.21ms
    2000x200@20perslice   19.70ms -> 19.76ms
    1v1-5000x50000        11.75ms -> 10.46ms

The deferred apply gives back part of the parallel win on the only workload
where rayon ever helped (1.12x here, against the 1.3x T3 reported), and the
sequential path is unchanged. Trading a fraction of a 1.3x speedup on one
pathological shape for the removal of undefined behaviour is the right side
of that bargain.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DnsaJg74eNSva3PJjK2eej

src/lib.rs

@@ -1,3 +1,5 @@
+#![forbid(unsafe_code)]
+
 use std::{
     cmp::Reverse,
     f64::consts::{FRAC_1_SQRT_2, FRAC_2_SQRT_PI, SQRT_2},