bug: let binding of Int from variable creates alias instead of copy

[rfunix]

Bug

let tmp: Int = some_var creates an alias to some_var rather than a value copy. Subsequent mutation of some_var changes tmp.

Repro

module alias_bug {
    meta { purpose: "repro", version: "1.0.0" }
    fn main() -> Int {
        let mut y: Int = 8
        let tmp: Int = y
        y = 4
        print_int(tmp)  // prints 4, expected 8
        print_int(y)    // prints 4
        return 0
    }
}

Expected output

8
4

Actual output

4
4

Notes

• Only affects let bindings from a plain variable name (e.g. let tmp: Int = y)
• Bindings from expressions work correctly: let r: Int = x + y is not affected
• Workaround: use an expression: let tmp: Int = y + 0
• Found during kodo-vericoding-bench development (v008 GCD iterative variant)
• Likely a MIR lowering or codegen issue where Int values are stored as pointers

[rfunix]

Fixed in PR #58. The root cause was in the MIR copy propagation pass ( in ): it only required the destination local to have a single definition before substituting, without checking the source. For mutable variables reassigned after the copy, this turned into an alias. The fix extends definition counting to all instruction types and requires both src and dest to have exactly one definition.