Bind app structure: TopAppBar family, TabRow family, Snackbar, Badge, ListItem

[jonathanpeppers]

Why

The Compose app-structure components — what most apps build their
chrome out of — are entirely missing:

• TopAppBar, CenterAlignedTopAppBar, MediumTopAppBar,
  LargeTopAppBar (androidx.compose.material3.AppBarKt)
• TabRow, ScrollableTabRow, PrimaryTabRow, SecondaryTabRow,
  Tab, LeadingIconTab (androidx.compose.material3.TabRowKt +
  TabKt)
• Snackbar, SnackbarHost, SnackbarHostState
  (androidx.compose.material3.SnackbarKt) — also needed by Scaffold's
  snackbarHost slot in Bind Scaffold and use it in the sample #12
• Badge, BadgedBox (androidx.compose.material3.BadgeKt)
• ListItem (androidx.compose.material3.ListItemKt)

What

Bind each via the same shape as the existing facades. Most of these
have stripped overloads (color-bearing @JvmInline value class params),
so they'll need raw-JNI bridges + declarative [ComposeDefaults]
attributes. Verify mangled names with javap against
Xamarin.AndroidX.Compose.Material3Android 1.4.0.3.

State holder: Snackbar needs SnackbarHostState. Check whether
SnackbarHostState is plain enough to use the bound C# constructor or
needs the same JNI-bridge treatment as DatePickerState /
TimePickerState from PR #9.

Sample

Wrap the per-tab content in a Scaffold (depends on #12) with:

• A CenterAlignedTopAppBar showing the current tab name as title
• The existing NavigationBar as bottomBar
• Replace the home-grown NavigationBar tab switcher with the proper
  TabRow + Tab API on at least one tab to demonstrate it
• Add a Snackbar triggered by a button on the Buttons tab