fix: remove string-based run_model API from C runtime + update main.c to use runModelWithBuffers

[chrispcampbell]

Fixes #302

@ToddFincannonEI: See issue for details. I don't know if you are relying on the string-based run_model API for EPS, but if you are, the changes to main.c demonstrate how to get the same behavior as before, so hopefully you could retrofit your code in a similar way. But let me know if any of this causes big problems for you.

I'm going to submit another PR shortly for changes that overlap slightly (adding parameters to runModelWithBuffers to support the "override constants" feature), so I suggest reviewing this one first. Once you approve both, I will merge this one first and will update the other to make the runModelWithBuffers changes consistent.

AI disclosure: I used Claude Code (Opus 4.5) to implement the first round of changes according to my detailed specification (recorded in the issue). I reviewed and edited the changes pretty heavily and rewrote the runModel[WithBuffers] docs for clarity.

[ToddFincannonEI]

I tested this by generating the arrays sample model before and after the changes to see what it's emitting. I don't have comments on the code gen implementation. The approach taken here looks good to me. The sparse input array will be important for EPS. It should be a relatively straightforward modification to replace our string-based input with the double buffer.