Concrete field types for every parametric law (breaking: type parameter)

[alecloudenback]

All 25 laws were Base.@kwdef struct with untyped fields, so no law was isbits and every hazard call boxed its parameters. Twenty of the 25 laws integrate hazard with quadgk, so this dominates the parametric path.

Measured here after the change: survival(Siler(), 20, 65) runs in 270 ns with 0 allocations (3.57 μs and 421 allocations on master).

Changes

• Each law is struct Law{T<:Real} <: ParametricMortality with field::T members and an explicit keyword constructor Law(; a=..., b=...) = Law(promote(a, b)...), so mixed keyword types promote (Makeham(a=1, b=0.5, c=0) is a Makeham{Float64}; Gompertz() is a Makeham{Float64}).
• T<:Real keeps ForwardDiff duals working for fitting.
• Defaults are unchanged in value. Weibull and InverseWeibull had all-integer defaults, which would construct as Law{Int}; they are spelled as floats now so every default law is Float64.

Testing

• For every subtype of ParametricMortality: L() is isbits, is L{Float64}, and hazard(L(), 50.0) infers as Float64.
• The existing comparisons against the R MortalityLaws fixtures are unchanged and pass.

---

Stacked on #139 (base branch refactor/04-split-parametric). Merge the stack in order; once the base PR is merged and its branch deleted, GitHub retargets this PR to master.

🤖 Generated with Claude Code

https://claude.ai/code/session_01JFnc6pvofuNXTW2xdCcewT

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.62%. Comparing base (8ab82cf) to head (f14b638).

Additional details and impacted files

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##           refactor/04-split-parametric     #140   +/-   ##
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  Coverage                         90.62%   90.62%           
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  Files                                10       10           
  Lines                               384      384           
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  Hits                                348      348           
  Misses                               36       36           

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