feat: Vary live region politeness based on priority

[smockle]

Fixes #27

This PR updates the polyfill so priority: normal messages go to an aria-live: polite live-region and priority: high messages go to an aria-live: assertive live-region, to more closely align with the spec and mappings.

[Copilot]

Pull request overview

Updates the ariaNotify() polyfill to select different live-region hosts based on message priority, aligning behavior with current spec direction for polite vs assertive announcements.

Changes:

• Split live region host selection into two custom element types: polite-live-region-* and assertive-live-region-*.
• Introduce PoliteLiveRegionCustomElement / AssertiveLiveRegionCustomElement subclasses to set aria-live appropriately.
• Update browser tests to assert routing via the correct live region host and that messages dispatch through it.

Show a summary per file

File	Description
arianotify-polyfill.js	Selects polite vs assertive custom live-region host based on message priority; defines dedicated custom element subclasses.
tests/web-test-runner/arianotify-polyfill.test.js	Reworks tests to validate correct host type/ariaLive and handleMessage dispatch for both priorities.

Copilot's findings

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Comments suppressed due to low confidence (1)

tests/web-test-runner/arianotify-polyfill.test.js:64

• This test uses a fixed 300ms delay before asserting handleMessage was called, while the polyfill uses a 250ms sleep internally. The small buffer can cause intermittent failures; prefer polling for the expected call (with a reasonable timeout) or increase the wait to provide sufficient headroom.

      await new Promise((resolve) => setTimeout(resolve, 300));
      expect(calls).to.have.length(1);

• Files reviewed: 2/2 changed files
• Comments generated: 1