Seam assert pre-scan misses assertions nested inside expressions

The seam-contract pre-scan collects assert(var == const) only from selected block-bearing expressions such as spawn, if, and loops. It skipped assertions inside ordinary expression containers.

Example:

fn f(value: i32) -> i32 {
    let result = 1 + if 1 == 1 {
        assert(value == 42);
        2
    } else {
        3
    };
    return result;
}

The assertion is valid Vx and should contribute the contract value == 42.

Related: https://github.com/vx-lang/Vx/issues/491

Start by tracing the seam-contract pre-scan logic that handles block-bearing expressions such as spawn, if, and loops, then reproduce the supplied function with an assertion nested in an expression container. Done means the nested assert(value == 42) contributes the seam contract just like assertions in the currently handled expressions.