feat(chat): stack the composer's header, chips and field

The composer layout photos need, without the photos. ChatInput becomes a
custom Layout (ComposerMeasurePolicy) that stacks an optional header row
(reply strip or photo chips) over the text field and lays out the
accessory and send button around it, matching the iOS composer.

- composerStacks() decides when the field goes multi-line; it is pure and
  covered by ComposerStackingTest.
- Height changes, the single-line to multi-line switch and the header
  arriving or leaving all ease on one spring (dampingRatio 1, ~0.28s, the
  iOS replySurface curve), so the field and its text move together.
- Corners stay concentric: the field's corner is the header chip's corner
  plus the inset, on one line or stacked.
- The header strip fades only at an edge it has scrolled past.
- Sizes come from the static grid; the one-line field is FieldPadding +
  AccessorySize + FieldPadding, the same 50dp as the $ button.

Author: bmc08gt