feat: rebuild in-app notifications as contained cards, round-square bot avatar - #29

A clipped back card (e.g. a news cover) ended exactly at the front card's
bottom edge, so its white body showed through the transparent notches behind
the front card's rounded bottom corners. Clip to the front card's height in
card space instead — after the peek offset and scale-back the clipped bottom
lands above the front card's bottom, matching the web widget's clip math.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

…o button state

Three refinements from live review:

- Shadows: card elevation drops 6dp -> 4dp and the close button 8dp -> 6dp,
  with the spot shadow tinted to ~60% black on P+ — soft and airy like the
  web widget's two-layer shadow instead of the heavier stock elevation look.

- Arrivals are choreographed as one deck motion: the previous cards animate
  back into their tuck while the new card emerges scaled-back from the
  stack's front slot, instead of the old front snapping into place and the
  new card floating up from the empty space below the stack. The very first
  notification keeps the web-style slide-up fade, and re-adds after an
  overlay rebuild (activity change, reconnect) no longer replay the
  entrance at all.

- The container's constraints re-apply when the feedback button's state
  flips (setShowFab): the button settles asynchronously after the config
  loads, and a container built in that window was anchored to the bottom of
  the screen — a notification arriving then rendered below the button until
  the next rebuild relocated it, which read as the card animating up from
  the screen edge.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

The arrival jank root cause: the stack frame was resized to the stack's
expanded height on every change. Cards are bottom-anchored, so the resize
re-based every card (and moved the container's top edge) while the arrival
animators were mid-flight — the whole deck rendered offset by the height
delta and visibly slid across the screen into place, reading as the new
card flying in from the bottom of the display.

The frame now keeps one fixed height (screen height, floor 1200dp), set
once at creation, so nothing ever re-bases: only the card animators move
cards. The container carries the same height explicitly in its constraints
— left at WRAP_CONTENT, ConstraintLayout measures it AT_MOST the parent and
the taller frame overflows past the container's bottom, pushing cards off
screen. The close button anchors to the stack's visual top computed from
the fixed height, and a newly added card is alpha-0 from the moment it
enters the hierarchy so it cannot flash at its resting position before the
entrance starts.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

DecelerateInterpolator(2f) front-loads ~60% of the travel into the first
frames — on a long tuck (a tall news card taking the front from a short
message) the deck appeared to teleport and then drift. All stack motions
(arrival choreography, expand, entrance, close button) now share the
material standard-easing curve (0.4, 0, 0.2, 1) at 350ms, matching the
gentle read of the iOS ease-out.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Two structural guarantees replace the per-animation fixes:

- The stack frame clips at its bottom edge permanently (generous side/top
  overscan for shadows and the close-button overhang, a small bottom
  allowance for the front card's shadow). No card, shadow or mid-animation
  overhang can ever draw below the stack — the feedback button underneath
  is untouchable by construction. Previously a tall card's unclipped body
  slid across the button during tuck/expand transitions, because the
  per-card clips only applied after the motion settled.

- Entrances materialize in place: the new front card (and the very first
  notification) fades in with a slight scale-up and zero translation, so an
  arrival can never read as a card traveling in from elsewhere on screen.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

The container-level clip guaranteed containment but guillotined the front
card's soft shadow at a straight line, and mid-animation a tall card's body
was visible (then bluntly cut) in the strip below the front card.

The web widget's model, ported properly: every card carries a clip at ALL
times — generous insets past the body in resting states (a visual no-op:
Android elevation shadows are outline-based and ignore clipBounds, so
nothing is ever cut), the front card's height for collapsed cards behind —
and every transition ANIMATES the clip in lockstep with the card's motion
(same duration, same curve, starting edge clamped to the card's body). The
sweeping edge mathematically never falls below the front card's bottom, so
a tall card can neither poke out below the stack mid-flight nor show a cut
edge: the clip line always hides behind the front card.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Elevation shadows are drawn from the view's outline and ignore the view's
alpha — a card (or the close button) fading in cast its shadow at full
strength from the first frame, a short dark flicker under a still-invisible
element. The card's elevation and the close button's now ramp from zero in
sync with their fades.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Elevation 4dp -> 3dp with a lighter spot tint (argb 115) — the barely-there
soft throw the iOS two-layer shadow has.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Re-syncs with the iOS SDK (which shipped 16.4.3 solo).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>