Bind Scaffold and use it in the sample

[jonathanpeppers]

Why

The follow-up sample from PR #9 added a NavigationBar for switching between
"Basics / Buttons / Pickers" tabs, but without Scaffold the bar flows
inline as a normal Column child instead of being pinned to the bottom of
the screen. It currently floats in the middle, which looks wrong and is
unrepresentative of how Material 3 apps are laid out.

What

Bind androidx.compose.material3.ScaffoldKt.Scaffold in the same shape as
AlertDialog (PR #6) — slot-style C# facade with named properties:

• TopBar (optional Function2)
• BottomBar (optional Function2)
• SnackbarHost (optional Function1)
• FloatingActionButton (optional Function0)
• content (the ComposableContainer body — required)

Scaffold's overload is stripped from the binding by the
@JvmInline value class color params (containerColor, contentColor),
so we'll need a raw JNI bridge in ComposeBridges.cs against the mangled
name (likely Scaffold-27mzLpw or similar — verify with javap against
the Xamarin.AndroidX.Compose.Material3Android 1.4.0.3 AAR), plus a
[ComposeDefaults("ScaffoldDefault", ...)] declarative attribute for the
$default bitmask.

Sample fix

In the same PR, update src/ComposeNet.Sample/MainActivity.cs to wrap the
existing per-tab content in a Scaffold and move the NavigationBar to
the BottomBar slot, e.g.:

new Scaffold
{
    BottomBar = new NavigationBar
    {
        new NavigationBarItem(selected: tab.Value == 0, onClick: () => tab.Value = 0)
        {
            Icon = new Text("🔢"), Label = new Text("Basics"),
        },
        // ...
    },
    Body = tabContent,
}

That makes the bottom bar look correct on the emulator (pinned to the
bottom edge above the system nav bar) and gives the sample a more
realistic Material 3 shape.

References

• Issue Bind dialogs and sheets: AlertDialog, ModalBottomSheet, BottomSheetScaffold, DatePickerDialog, TimePicker, Tooltip #3 — original dialogs/sheets issue (closed)
• PR Bind Material 3 AlertDialog (slot-property pattern) #6 — AlertDialog template (slot-property facade pattern)
• PR Bind ModalBottomSheet, BottomSheetScaffold, date/time pickers, and Tooltip #9 — added NavigationBar, currently mis-positioned in the sample
• Material 3 Scaffold docs:
  https://developer.android.com/reference/kotlin/androidx/compose/material3/package-summary#Scaffold