prep: extend C stubs to APIs with instride inputs

[ferronn-dev]

Summary

• Adds instride support to the C stub generator in prep.lua, matching the behavior already present in the stubby Lua stub generator
• Fixed inputs (1..nsins) are checked at their literal stack positions as before
• Stride inputs are checked inside a C for loop (for (int i = nsins+1; i <= top; i += instride))
• Extracts an emitinpcheck helper to avoid duplicating the type→function-name mapping between fixed and stride paths

Test plan

• All existing tests pass (cmake --build --preset default --target test)
• No APIs currently qualify as C stubs with instride (all existing instride APIs have ineligible types like unknown/table), but the generator is ready for when they do

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