Exercise 2: Passing a Binding<String> to a Child TextField — Possible Solution
=====================================================================================

struct ParentView: View {
    @State private var username = ""

    var body: some View {
        VStack {
            UsernameField(username: $username)
            Text("Current value: \(username)")
        }
    }
}

struct UsernameField: View {
    @Binding var username: String

    var body: some View {
        TextField("Enter username", text: $username)
    }
}

HOW IT WORKS: ParentView owns the real, persistent state - a
@State private var username - and passes $username, the projected
Binding<String> value (not the plain String itself), down to
UsernameField's own username parameter, which is declared as
@Binding var username: String to receive exactly that kind of
two-way connection rather than a plain copy.

Inside UsernameField, the TextField is itself bound to $username -
using the $ prefix a second time, since @Binding properties also
expose their own projected value, allowing the binding to be passed
along yet further if needed. Typing into the TextField writes directly
through this chain of bindings back to ParentView's own real,
externally-stored username state (per Exercise 1's own @State
mechanism) - there's no copy anywhere in this chain breaking the
connection, so ParentView's own Text showing "Current value:
\(username)" updates live as the user types.

ANSWER: ParentView holds @State private var username and passes
$username to UsernameField's @Binding var username, which in turn
binds a real TextField to $username. Because a Binding is a genuine
two-way connection rather than a copy, typing into the TextField
updates ParentView's own username state directly, confirmed by the
parent's own Text view reflecting the change immediately.

WHY THIS WORKS AS AN ANSWER
------------------------------
This correctly implements a two-level State-to-Binding chain and
explains why the connection stays live end to end, rather than merely
asserting it works.