Add missing prototypes to user functions in sdlib.c

[sherm1]

Previously sdlib declared user function pointers as int (*func)() but then called func with arguments. That's deprecated in C and an error in C++. This PR modified sdlib.c code generation to define the function prototypes properly.

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This change is 

[Copilot]

Pull request overview

This PR updates the SDFast code generator’s handling of user-supplied function pointers so generated C/C++ code uses proper function pointer prototypes (instead of deprecated/invalid “unspecified arguments” forms).

Changes:

• Add prototype strings for generated VT_PROCNAME parameters in root-finding and integrator helpers.
• Extend the packedvar_t / var-decl machinery to carry and print function pointer prototypes for C-family languages.
• Update declare_proc / packvar call sites to pass the prototype where needed.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

File	Description
sdfast_main/sdroot.c	Adds a shared prototype string for root callback function pointers and passes it through packvar.
sdfast_main/sdinteg.c	Adds an integrator callback prototype string and uses it in declare_proc/packvar declarations.
calc/decl.c	Adds storage/varargs handling for VT_PROCNAME prototypes and prints C-family function pointers with explicit prototypes.
calc/calc.h	Extends packedvar_t with a pv_prototype field for procedure parameters.

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[sherm1]

@sherm1 reviewed 5 files and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on sherm1).

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

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[sherm1]

@sherm1 reviewed 2 files and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on sherm1).

[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

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[Copilot]

This introduces a declaration (char *prototype = NULL;) after an executable statement (vname = ...). If this project is compiled in C89/gnu89 mode (common for SD/FAST-era code), mixed declarations/statements can fail or at least warn. Move the prototype declaration up with the other locals (and just assign/reset it inside the loop) to keep the file C89-compatible and consistent with the surrounding style.