feat(tipping): fade the Chats list under its title bar

[bmc08gt]

The Chats list was cut off at the title bar instead of passing under it, unlike the chat screen's message list, which dissolves into the bar's scrim as it scrolls.

CodeScaffold could not express that placement. Material 2's Scaffold shortens its body by the bar heights and places it between them, so the list can never run underneath; the chat screen worked around it with a private ChatInputScaffold in the messenger module.

CodeScaffold now takes a ScaffoldBarPlacement. In Overlay the Scaffold's own bar slots are left empty and the bars go to a subcomposed layout inside the content slot: the bars are measured first, the content is measured at full size and placed at (0, 0), and the bars' heights come back as the PaddingValues the content already receives. Both are measured in the same pass, which is what keeps the content correctly padded on the first frame — onSizeChanged hands it zero on frame one and snaps on frame two, which the message list showed as a jump on every open. Background, snackbar host, drawer and FAB are untouched, and Inset remains the default, so no existing caller moves.

The Chats tab uses Overlay with a background-to-transparent scrim behind AppBarWithTitle. The scrim is drawn with drawBehind so its 48dp reach past the bar's bottom edge stays out of the bar's measured height, which is what the list pads by: rows begin dissolving below the title and still come to rest clear of it. The list's own start-edge gradient is dropped, since the scrim now does that work and two fades darken rows twice over.

MessengerScreen moves onto the same scaffold and its private one is deleted.

ChatsTabLayoutTest pins the contract: the list spans the full height between the bars, and the first row rests below the top bar.