New wall-clock timing fields added to turn telemetry

What

The telemetry sent with turn results gained several new optional timing fields to break latency down into stages:

• first_stream_post_queue_wait_ms and first_stream_post_queued_behind, showing queueing latency and what it was queued behind.
• frame_received_wall_ms and frame_enqueued_wall_ms, wall-clock timestamps for when a frame arrived and was queued.
• turn_started_wall_ms, marking when the turn started.
• first_text_post_ms and first_text_post_wall_ms, marking the first text output.

These fields are populated for sessions that read input from an SSE stream via --sdk-url, including headless/hearth-relay turns.

Why

Breaking turn latency into transit, queueing, and processing stages makes it possible to diagnose where time is actually going in a slow turn, rather than only seeing total latency.