Move Toast transitions to CSS with @starting-style

[mdo]

Toast was the last component still using the v4-era show dance: JS toggled .fade, forced a reflow, then juggled .show, .showing, and a deprecated .hide class to fake an exit animation. Menu, Dialog, and Drawer already express this in CSS.

• Move the fade onto .toast itself, with @starting-style for the entry state and a discrete display transition so the toast stays laid out while it fades out. JS now only toggles .show.
• Add --toast-transition-duration and --toast-transition-timing tokens.
• Remove the animation option and data-bs-animation. Add .toast-instant to skip the animation, matching .dialog-instant and .drawer-instant.
• Remove the .showing class, the deprecated .hide class, the .fade toggle, and the reflow() call from toast.ts. Alpha is the right time to drop these rather than ship them.
• isShown() now returns false as soon as hide() is called, rather than when the fade-out ends. This is inherent to allow-discrete and is noted in the migration guide.
• Drop the now-dead showing class from the PurgeCSS safelist in the optimize docs.

Verified in Chromium against the built bundle, since the unit tests run without stylesheets loaded. The animated toast holds display: flex at opacity 0.61 partway through the fade-out and fires hidden.bs.toast at 146ms; .toast-instant reports 0ms for both events. Under prefers-reduced-motion: reduce the duration drops to 0s and the events still fire through the existing executeAfterTransition fallback.

Supersedes #42101, which proposed replacing the autohide timer with a CSS animation driven by --bs-toast-delay. That approach turned autohide: false into a 99999s sentinel, fired shown/hidden synchronously, and changed pause-on-hover from restart to resume. This PR takes the transition modernization from that proposal and leaves the autohide timer in JS.