Function walk_steps_par 

pub fn walk_steps_par<Y, E, F>(
    walk_params: &WalkParams<Positive, Y>,
    next_y: F,
) -> Result<Vec<Step<Positive, Y>>, E>
where
    Y: TryInto<Positive> + Display + Clone + Send,
    E: From<SimulationError> + Send,
    F: Fn(&Positive, Option<Positive>, &Xstep<Positive>) -> Result<Option<Y>, E> + Sync,

Parallel variant of walk_steps: identical contract and output, with the per-step next_y calls executed on the rayon thread pool.

The walk itself (walker output, per-step volatilities, x-step sequence) is computed serially and is identical to walk_steps; only the y-value construction — the dominant cost when next_y rebuilds an option chain or series — is fanned out. Output is deterministic and equal to the serial driver’s for the same inputs because every per-step input is precomputed and order is preserved on collection.

Sequential/parallel equivalence

• The price path is drawn once, serially, from a single generator before anything is fanned out: deterministic_rng(seed) when WalkParams::seed is Some(seed), the thread RNG otherwise. No random number is drawn on a rayon worker, so the thread count and the scheduling cannot reach the random stream.
• Nothing is reduced across steps in parallel; the y-values are assembled in step order, so no Decimal or f64 result depends on the order in which workers finish.
• Hence, for a seeded walk and a next_y that is a pure function of its arguments, walk_steps_par(p, f) equals walk_steps(p, f) bit for bit, on every run.
• An unseeded walk draws a new path on every call, serial or parallel, so two calls agree only in distribution.

Semantics notes versus the serial driver:

• next_y must be Fn + Sync (stateless or internally synchronized) instead of FnMut.
• Ok(None) still ends the walk at the previous step; steps past the first None are computed speculatively and their results — values AND errors — are discarded, exactly as the serial driver never evaluates them.
• On failure, the first error in step order that precedes any stop signal is returned.

Errors

Same as walk_steps.