DSLX allows parameterized struct assignments with invalid width fields

A struct with parameterized width-sized fields is not properly deduced when assigned from `TYPE { field-assignments...}`. Even if a particular type is requested (below: `Foo<5>`), the width-deduction happens on the rhs and then is never checked against the requested type.

```rust
pub struct Foo {
a: uN[WIDTH],
}

fn sizeof_foo(x: Foo) -> u32 {
WIDTH
}

#[test]
fn struct_assign_test() {
let x = Foo<5> { a:u17:1 }; // Want type Foo<5>, but with undetected typo ...
assert_eq(sizeof_foo(x), u32:17); // ... so actually got type Foo<17> instead
}
```

**Expected**: compile error pointing out the mismatch.

Start with the DSLX struct_assign_test example in the issue and reproduce its compilation behavior. The completed fix should reject Foo<5> { a:u17:1 } with a compile error identifying the mismatch between the requested parameterized type and the deduced field width.

Written by the indexing model from the issue text.