feat(Dialog): enter and exit animations (#613)

[mathewtaylor]

Closes #613.

What changes

BbDialog and DialogOpenOptions (for DialogService.OpenAsync) take five new options, built on Motion's presets:

• EnterPreset / ExitPreset (MotionPreset?): the preset played on open, and played in reverse on close, matching how BbMotion hides content.
• EnterKeyframes / ExitKeyframes: custom Web Animations keyframes. These override the preset, and exit keyframes play as written.
• AnimationDuration (ms, default 200).

A closing dialog stays on the page, with data-state="closed", until its exit animation ends. Focus trap, scroll lock and the native <dialog> top layer are released after that. Reopening mid-exit cancels the exit. With no options set, dialogs open and close exactly as before. Reduced motion skips the animations, as Motion already does. For service dialogs, the OpenAsync result still completes at once; only the removal waits.

How

• Primitives: DialogContext gains an internal "exiting" phase. Portal, overlay and content render while the dialog is open or exiting. BbDialogContent (primitive) gets two public hooks, EnterAnimation and ExitAnimation (Func<ElementReference, Task>?). The exit hook is what holds the dialog on the page.
• Components: motion.js gets a one-shot transition export that reuses presetFrames. The styled BbDialogContent and BbDialogProvider call it. The provider keeps a closing dialog in its stacking position until the exit ends.
• When a Motion enter is set, the built-in zoom is dropped. The CSS fade-in stays so the content doesn't flash before the script starts.

Demo and API

• New "Animations" section on the Dialog demo, with the animations.txt snippet.
• API Reference entries added for Dialog and DialogOpenOptions.
• API surface snapshots updated: five Dialog parameters, the private cascade, and the two primitive hooks.

Tests

• DialogAnimationTests (8 bUnit tests): no-animation dialogs still leave at once; exit holds the dialog until the animation ends (uncontrolled, controlled, service); reopen mid-exit; enter preset plays and replaces the zoom.
• tests/js/motion.test.mjs: 3 new tests for transition (reversed preset with held last frame, reopen drops the held exit, custom exit keyframes, reduced motion).

Note

#612 (position, #614) changes the same Dialog files. Whichever merges second needs a small rebase.