cgen: fix pointer reference in for-in fixed-array iteration with & [fj4WqyCCw3C5ShR1RfB7MoBPTpkRrBFYP1uT35g3MvT] - #28166

Fix for i in &fixed_array producing invalid C when the array element is a struct

Description

When iterating over a fixed array by reference (for i in &t.buf where t.buf is a fixed array of structs), the cgen generates main__Item* i = (*tmp)[idx] — assigning a struct value to a pointer type.

Root Cause

In vlib/v/gen/c/for.v, the .array_fixed branch computes the address prefix as:

addr := if node.val_is_mut { '&' } else { '' }

This only checks val_is_mut, but not val_is_ref. When val_is_ref is true (the user wrote for i in &expr), the loop variable is a pointer type (&Item), but the generated C code assigns the element's value instead of its address.

Fix

Change the condition to also check val_is_ref:

addr := if node.val_is_mut || node.val_is_ref { '&' } else { '' }

Reproduction

module main

struct Item {
    a [100000]f32
}

@[heap]
struct Test {
    buf [10]Item
}

fn main() {
    t := Test{}
    for i in &t.buf {
        println(i.a[0])
    }
}

Before: error: initializing 'main__Item *' with an expression of incompatible type 'main__Item'
After: compiles correctly.

Closes #28128
[fj4WqyCCw3C5ShR1RfB7MoBPTpkRrBFYP1uT35g3MvT]