Add barge-in for realtime voice playback

[rogerchappel]

Summary

• Adds a realtime relay cancelOutput action and maps it to OpenClaw talk.session.cancelOutput
• Adds client helper coverage for cancelling realtime output
• Detects speech while assistant audio is playing, stops local playback immediately, and sends barge-in cancellation to the gateway

Why

Realtime audio exchange works, but speaking over the assistant did not interrupt live playback. This makes realtime voice behave closer to a normal conversation.

Local verification

• pnpm test -- src/lib/gateway-client-realtime.test.ts 'src/app/api/runtimes/[id]/talk/realtime/relay/route.test.ts' src/lib/realtime-voice-client.test.ts src/lib/realtime-voice-audio.test.ts
• pnpm typecheck
• git diff --check origin/main..HEAD
• pre-push: pnpm typecheck && pnpm build

Manual test notes

With NEXT_PUBLIC_CREWCMD_REALTIME_VOICE=1, start realtime voice and speak while the assistant is talking. Expected behavior:

• current assistant audio stops immediately in the browser
• /talk/realtime/relay receives action: "cancelOutput"
• the next user utterance continues through the realtime relay without falling back to /api/stt