fix(snackbar): make lifecycle dismissal deterministic

[Banegasn]

Summary

• Bind snackbar exit teardown to cancellable animation completion rather than an untracked delay.
• Restart auto-dismiss on active duration changes, cancel lifecycle work on disconnect, and define exactly-once dismissal reasons.
• Document reopen/replacement behavior and add deterministic fake-clock/event-driven browser races.

Acceptance mapping

• Reopening during exit cancels the stale lifecycle and remains open.
• Disconnect clears the auto-dismiss timer and aborts exit completion listeners.
• Changing duration while open restarts the full auto-dismiss interval; changes during exit do not alter that dismissal.
• snackbar-dismiss is emitted once per dismissed lifecycle with timeout, action, programmatic, or replacement detail.
• open changes after animationend/animationcancel, or immediately when reduced motion removes animation.
• Browser-matrix race tests use fake timers and dispatched animation events; no arbitrary delays.

Validation

• pnpm lint
• pnpm typecheck
• pnpm test (including Chromium, Firefox, and WebKit)

Fixes #24