[Bug] Snackbar: no alert-role path and unreliable first announcement

[johnleider]

From the #543 WS4 audit. Two related live-region gaps:

1. SnackbarRoot.vue:81 hardcodes role=status while the header comment (:8) promises "alert for urgent, status for informational" — no prop exists to choose. (~5 LOC prop.)
2. The live-region node mounts with its text already inside, instead of the empty-then-fill pattern; many SRs miss the first announcement. PaginationStatus.vue:74-106 implements the correct deferred pattern (100ms empty-then-fill) to mirror.

Focus is correctly never stolen (non-modal portal, scrim:false SnackbarPortal.vue:67) and the close button is labeled (SnackbarClose.vue:61-62).

[johnleider]

PR #624 lands part 1 of this issue: the `urgent` prop that switches the live region to `role="alert"`.

Part 2 is still open and this issue should stay open after #624 merges: the live-region node still mounts with its text already inside, instead of the empty-then-fill (deferred ~100ms) pattern, so many screen readers miss the first announcement. Mirror the correct deferred pattern in PaginationStatus.vue:74-106.

[johnleider]

Reopening. #624 (merged) addresses the alert-role half of this issue — SnackbarRoot now takes an urgent prop that switches the live region to role="alert" (assertive) vs the default role="status" (polite).

The unreliable first announcement half remains open: the live-region element must be present in the DOM before its text content is inserted, otherwise assistive tech misses the first message. Tracking that here.