feat(ui): add accessible AudioPlayer

[bntvllnt]

Closes #473

Summary

• add a URL-backed AudioPlayer with native playback, seek, time labels, optional transcript, captions, and rate selection
• scope playback keyboard shortcuts to the playback button and preserve native slider controls
• publish the component through the registry with its a11y keyboard map and committed visual baseline

Verification at 19853c00d9e4eb40190e3706e2741b81c085ee1c

• pnpm -F @vllnt/ui exec eslint src/components/audio-player/audio-player.tsx src/components/audio-player/use-audio-player.ts src/components/audio-player/audio-player.test.tsx src/components/audio-player/audio-player.visual.tsx (pass; visual fixture is intentionally ignored by ESLint)
• pnpm -F @vllnt/ui test:once -- audio-player.test.tsx (pass: 319 files, 1678 tests)
• pnpm -F @vllnt/ui exec tsc --noEmit --project tsconfig.build.json (pass)
• pnpm -F @vllnt/ui exec tsx scripts/check-story-coverage.ts and pnpm -F @vllnt/ui exec tsx scripts/verify-stories.ts (pass: 314 components)
• pnpm -F @vllnt/ui-registry registry:integrity (pass: 314 items)

Remaining gates

• Draft pending independent review, exact-head CI, and the required workspace build/lint/test gates.
• A committed Chromium baseline is included. A repeat local visual run was not performed because a shared Playwright server already occupied port 3100; it was not terminated or reused.

[vllnt-pilot]

ntk preview

App	Status	Preview
ui-registry	Failed: build ui-registry: build failed: exit status 1	—
storybook	Failed: build storybook: build failed: exit status 1	—

Built from 19853c00 · public + no-index (ADR-082) · torn down on close