feat(Utilities): add safe_call for type-inferred conditional fallbacks

[haakon-e]

Summary

Add Utilities.safe_call, a higher-order helper for conditional values whose fallback type is inferred from the computation rather than from its inputs:

safe_call(q_ice > ϵ; otherwise = zero) do
    n_ice / q_ice
end

It returns the do block when cond is true and otherwise(FT) when it is false, where FT = Core.Compiler.return_type(f, Tuple{}).

Motivation

The recurring zero(promote_typeof(args...)) idiom has a weakness that safe_call removes:

• promote_typeof types the fallback from the inputs, and every derivative-carrying argument must be listed by hand; omitting one returns a Union, which is non-concrete when using ForwardDiff on the GPU. safe_call takes the type from the function's range, so it cannot omit an argument.

Notes for review

• Core.Compiler.return_type is internal and inference-directed: the fallback type is correct only where the computation is type-stable; this is required for the code to run on GPUs anyways. Where inference widens (Union type), this will fail.
• No call site is converted here, though I have tested it on _regularised_ratio in the same module. With that, it is type-stable and allocation-free through ForwardDiff on CUDA, for the default and overridden otherwise.

[haakon-e]

This change is part of the following stack:

• Make the pointwise 2M+P3 tendencies ForwardDiff-differentiable #725
  • feat(Utilities): add safe_call for type-inferred conditional fallbacks #746 ◀

_{Change managed by git-spice.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.97%. Comparing base (ddd64f0) to head (78b754c).

Additional details and impacted files

@@                 Coverage Diff                 @@
##           he/ad-compat-2m-p3     #746   +/-   ##
===================================================
  Coverage               92.96%   92.97%           
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  Files                      56       56           
  Lines                    2859     2862    +3     
===================================================
+ Hits                     2658     2661    +3     
  Misses                    201      201           

Components	Coverage Δ
src	93.78% <100.00%> (+<0.01%)	⬆️
ext	69.47% <ø> (ø)

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[haakon-e]

Closing in favor of branch-free patterns: rate functions are now evaluated unconditionally, with ifelse selecting the result, rather than short-circuiting on a condition (see e.g. #757). This removes the use case for safe_call.