Fix compile error in AT_lambda_Vavilov_FWHM under GCC 16 / C23

[grzanka]

Summary

AT_lambda_Vavilov_FWHM in src/AT_EnergyLoss.c called AT_lambda_Landau_FWHM_left(kappa, beta), but AT_lambda_Landau_FWHM_left is declared and defined with no parameters. Under C17 and earlier, an empty parameter list () in a non-prototype declaration meant "unspecified arguments," so compilers tolerated the extra arguments. C23 (the default standard in GCC 16) changes this: () now means the function takes exactly zero parameters, so the call becomes a hard compile error ("too many arguments to function").

The correct counterpart to AT_lambda_Vavilov_FWHM_right(kappa, beta) is AT_lambda_Vavilov_FWHM_left(kappa, beta), which already exists in the same file and takes kappa/beta — it was simply the wrong function being called.

// before
double AT_lambda_Vavilov_FWHM(const double kappa, const double beta) {
    return (AT_lambda_Vavilov_FWHM_right(kappa, beta) - AT_lambda_Landau_FWHM_left(kappa, beta));
}

// after
double AT_lambda_Vavilov_FWHM(const double kappa, const double beta) {
    return (AT_lambda_Vavilov_FWHM_right(kappa, beta) - AT_lambda_Vavilov_FWHM_left(kappa, beta));
}

This also fixes a latent logic bug: AT_lambda_Vavilov_FWHM was mixing a Vavilov-distribution FWHM-right value with a Landau-distribution FWHM-left value (a constant, independent of kappa/beta), instead of using the matching Vavilov-distribution left value.

Test plan

• Reproduced the exact compiler diagnostic ("too many arguments to function") with a minimal repro compiled under -std=c2x (C23 semantics), confirming this is the root cause of the GCC 16 failure.
• Compiled src/AT_EnergyLoss.c standalone with gcc -std=c2x after the fix — no errors.
• Full CMake/CI build (not run locally in this environment; will be verified by CI on this PR).

Fixes #287

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Generated by Claude Code

[Copilot]

Pull request overview

This PR fixes a C23/GCC 16 compile error (and a matching logic bug) in AT_lambda_Vavilov_FWHM by replacing an incorrect call to the zero-argument AT_lambda_Landau_FWHM_left() with the correct two-argument AT_lambda_Vavilov_FWHM_left(kappa, beta).

Changes:

• Corrected AT_lambda_Vavilov_FWHM to compute the Vavilov FWHM width as right - left using Vavilov endpoints.
• Eliminated a C23 “too many arguments to function” error by removing arguments from the Landau-left function call (by no longer calling it there).

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