fix(announcer): keep aria live-region label until next announcement (fixes #31)

[chiefcll]

Fixes #31.

Problem

In aria mode (Announcer.aria = true), focusElementForAria() injected the focus label into the #aria-parent aria-live="assertive" region, then a hard-coded setTimeout(…, 100) deleted those nodes and moved focus back to the canvas. On webOS/Samsung TV screen readers (Audio Guidance), that teardown fires before the reader has finished — often before it has started — so nothing is announced, even though the DOM mutates exactly as expected. The cancel() path did the same immediate clear + focusCanvas().

Fix — proposed option #1 (replace-on-next-write)

The live region now holds the current label until the next announcement replaces it, instead of being torn down on a timer:

• focusElementForAria() — removed the 100ms teardown timer. The region is cleared (cleanAriaLabelParent()) immediately before the new spans are injected, so it always contains exactly the current label and the reader is never interrupted. Added an early return when there are no phrases to write, so a canceled/empty series can't blank out a label the reader is still speaking.
• cancel() (aria path) — no longer clears the DOM region or moves focus. It drops any partially-accumulated phrases and falls through to actually stop the series (active = false, cancel nested) instead of returning early.
• Removed the now-unused focusCanvas() helper — no focus is moved at all anymore, which also avoids focus changes interrupting the reader.

Tests

Added tests/announcer-aria.spec.ts covering:

• label is injected into the assertive live region;
• label survives well past the old 100ms window;
• the next announcement replaces the prior label (region holds only one);
• a canceled follow-up series does not wipe the region.

All 4 pass; tsc, ESLint, and Prettier are clean. (The pre-existing flex.performance.spec.ts timing benchmark can flake under load and is unrelated to this change.)

Reviewer note

This slightly changes cancel() semantics in aria mode: the series now actually halts (active = false / cancels nested) instead of returning early. That is more correct, but flagging in case anything relied on the old early-return behavior.

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