Function coerce_user_fn_args 

pub(crate) fn coerce_user_fn_args(src: &str) -> String

Collect every top-level user-defined function in src, then walk every call site to those names and coerce mismatched arg types. MD2 author sloppiness picked up here: passing lavcol(ret*2) where ret*2 is vec3 but lavcol(float t) expects scalar (the EVET preset). HLSL silently truncated; WGSL refuses with InvalidCall.

Coercion rules per arg:

• vec_n → f32: truncate via .x (single-component swizzle).
• f32 → vec_n: broadcast via vec_n<f32>(arg).
• vec_n → vec_m where n > m: truncate via .xy/.xyz.
• vec_n → vec_m where n < m: pad via vec_m<f32>(arg, 0.0, …).
• any other mismatch (Unknown, mat*, bool): pass through.

Conservative: only fires when the param type is a known scalar/vec and the inferred arg type is a known scalar/vec. Anything ambiguous stays untouched.