Requires SciML/ModelingToolkit.jl#4274 and an MTKBase release before we can test/merge.

Summary

Adds @poissonians as a DSL option in @reaction_network and @network_component, allowing users to declare poissonians inline rather than constructing them externally and interpolating.

rn = @reaction_network begin
    @parameters λ k d
    @variables X(t)
    @poissonians dN(λ)
    @equations D(X) ~ dN
    (k, d), 0 <--> S
end

Multiple poissonians and begin/end block syntax are supported:

@poissonians begin
    dN₁(λ₁)
    dN₂(λ₂)
end

Rate expressions can also be interpolated from external variables:

rate_expr = k * V
rn = @reaction_network begin
    @poissonians dN($(rate_expr))
    ...
end

Note: Symbols appearing in poissonian rates must be pre-declared (via @parameters, @species, etc.) or interpolated — they are not auto-inferred.

Depends on: A fix in ModelingToolkitBase that makes @poissonians consistently return Vector{Num} (matching @brownians behavior). Without that fix, the macro returns a scalar Num for a single poissonian and a Tuple for multiple.