feat(stt): interactive VAD events demo on the feature page

[abhishekmishragithub]

Stacks on top of #271. Adds an interactive React component to the VAD events page that lets readers play sample audio and see the captured server message stream in sync.

What ships

File	Purpose
`fern/components/vad-events-demo/VadEventsDemo.tsx`	React component. SVG waveform + native audio playback + scrolling event log. ~16 KB.
`fern/components/vad-events-demo/assets/{clean,multi-turn,no-tail}.mp3`	Three audio samples. ~210 KB total.
`fern/components/vad-events-demo/fixtures/{clean,multi-turn,no-tail}.json`	Captured live event streams + precomputed amplitude arrays. ~20 KB total.
`scripts/spec-live-tests/prep_vad_demo_fixtures.py`	Reproducer. Downloads source audio, builds three variants with ffmpeg, runs each through the live Pulse STT WebSocket, writes fixtures.
`fern/products/waves/.../vad-events.mdx` + mirror	Adds the import and a `` block under a "Try it" section.

Three fixtures, three behaviors

Fixture	speech_started	speech_ended	Transcripts	Illustrates
clean	1	1	7	Normal single-utterance case.
multi-turn	2	2	13	Per-voiced-region behavior across two turns.
no-tail	1	0	6	The "speech_ended requires trailing silence" caveat from Notes.

Every event payload in the fixtures comes directly from the production endpoint. The component does not connect to a WebSocket at runtime; everything is static.

Behavior

• Sample picker at the top switches between fixtures. Each chip shows a title and a one-line subtitle.
• Waveform is rendered as 200 SVG bars from a precomputed peak-amplitude array. Vertical dashed markers sit at the timestamps of every `speech_started` and `speech_ended` event.
• Play button drives a native `` element. A live playhead moves across the waveform.
• Event log scrolls in sync. The row whose timestamp matches the playhead highlights and auto-scrolls into view. Earlier rows fade in (full opacity once "reached"), later rows are dimmed.

Why this design

Two ideas, one demo:

1. Timing: the waveform + markers make the acoustic-boundary concept tactile. The reader hears the sample audio and sees where the events fire on the wave.
2. Wire shape: the event log shows the actual JSON each customer would receive, with the same discriminator field they'd switch on in their code.

The Mermaid sequence diagram below the demo is the formal contract; the interactive demo is the hands-on layer above it.

Verification

• `fern check`: 0 errors.
• Live fixture regeneration verified: all three variants captured with expected event counts. Replayable via `SMALLEST_API_KEY=... python3 scripts/spec-live-tests/prep_vad_demo_fixtures.py`.
• v4-mirror diff: empty.
• llms.txt regenerated; in sync.

Test plan

• `fern check` green.
• Render check on Vercel preview: confirm the component renders, the play button plays audio, markers display at the right positions, event log highlights in sync.
• Confirm preview works on both desktop and mobile (the waveform SVG uses `preserveAspectRatio="none"` so it should scale; the fixture chips wrap on narrow screens).