feat(ui): animate session switching — pill reveal + bubble pop

[itsdestin]

DRAFT — do not merge. This needs another session of refinement first. See
"Outstanding before merge" below. It is opened now to capture the work and the
reasoning while it's fresh, not because it's ready.

Switching conversations was two hard cuts: the active pill's name snapped open,
and the transcript was replaced instantly. This adds motion to both.

The pill had two independent reasons it couldn't animate

Worth recording, because "just add a transition" was the wrong diagnosis — one
already existed and was being suppressed.

1. maxWidth was undefined when the pill was active (SessionStrip.tsx:785-787),
   leaving no numeric pair for the browser to interpolate, so width snapped to
   intrinsic.
2. transition was hard-set to 'none' for every pack-expanded pill
   (:789) — and packSessions puts the active pill in expanded
   unconditionally (pack-sessions.ts:53). The pill you just clicked was
   precisely the one with animation switched off.

Fixed by animating grid-template-columns: 0fr → 1fr, which interpolates to the
label's intrinsic width — max-width can't do that without imposing a hard cap,
and the active pill is meant to be uncapped so it flex-shrinks and ellipsizes only
when the strip itself is narrow. Non-active hover reveals keep their 120px cap.
The 'none' is preserved for repack churn and overridden only inside a short
window armed by an active-id change.

Bubbles

bubble-switch-out / bubble-switch-in keyframes on the same overshoot curve the
pills already use (cubic-bezier(0.34, 1.56, 0.64, 1)), 30ms stagger capped at 8.

Applied only to .timeline-entry.in-view — reusing the class the bubble
IntersectionObserver already maintains for the glass optimization
(ChatView.tsx:273-291). Off-screen bubbles are never animated, so cost stays flat
regardless of transcript length. This was a hard requirement from the design review.

Panes stay mounted and stacked, so the outgoing one is held for 120ms to finish its
exit while the incoming bubbles wait out an equal animation-delay with both
fill — sequential, not a crossfade.

Both keyframes end on transform: none rather than scale(1), matching the
row-fade-in convention (globals.css:1168-1171) that avoids leaving a persistent
compositing layer under fill-mode: both.

Scope

• Gated on prefers-reduced-motion and the app's reducedEffects setting.
  The session strip had no reduced-motion gate at all before this.
• Desktop only, by nature. Android's forceSingle filters to one pill keyed by
  session id, so it remounts on every switch — no before-state to transition from,
  and no expansion to show, since its single pill is always named. Not faked.
• Buddy window's SessionPill is a plain dropdown with always-visible labels; the
  pill fix does not apply there and it is untouched.

Outstanding before merge

1. The 120ms exit needs a real-use verdict. It is the cost of a genuine
   "disappear" instead of a cut, and it delays the incoming conversation by 120ms.
   Eyeballed in a dev instance and it reads fine, but it hasn't been lived with.
   Cutting the exit half is a one-line change if it grates.
2. **It also fires on Ctrl+** (chat↔terminal). A ChatView's sessionIdnever changes, sovisible` is the only edge available and the view toggle is
   indistinguishable from a session switch at this layer. Suppressing it needs a
   different signal threaded down from App.
3. No test coverage. Nothing in the suite renders SessionStrip or ChatView
   (only packSessions' 9 pure cases are pinned), so nothing here is guarded
   against regression. Worth deciding whether the .in-view-only rule — the load
   bearing perf property — deserves a pinning test.
4. Interaction with drag-reorder is unverified. The pill's drag path sets its
   own transition and suppressClick; the activeSwap window has not been
   tested mid-drag.
5. Not verified on a narrow viewport or over remote access, where the strip
   packs differently.

Verification

tsc --noEmit clean; full suite green (2784 passed, 1 skipped file). Eyeballed in a
dev instance on run-dev.sh across themes. Interactive verification was handed to
Destin per the workspace rule rather than scripted.

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