[P2-02] Add voice, realtime, and audio streaming support

[erikhoward]

Goal

Extend PetalFlow beyond text/image/file workflows for realtime and audio agent experiences.

Scope

• Define audio input/output artifacts and realtime event semantics.
• Support session lifecycle, interruption, backpressure, cancellation, and reconnect.
• Add provider capability negotiation and latency/quality telemetry.
• Define privacy, retention, and content-type limits for audio data.

Acceptance criteria

• A realtime workflow can start, stream, interrupt, and close a session deterministically.
• Audio payloads follow the artifact lifecycle and authorization policy.
• Provider-specific features do not leak into the core contract without capability checks.
• Reconnect, cancellation, malformed media, and backpressure are tested.

Priority

P2 — valuable post-1.0 capability.

See the PetalFlow 1.0.0 Readiness Report for the audit context.