v2: Session tracing

[loyality7]

Context

When a task takes longer than expected, there's currently no way to see why beyond ad-hoc logging. A timeline of navigations, requests and actions for one session makes that visible.

Scope

• Per-session timeline: navigation events, requests, input actions, script evaluations, each timestamped.
• Exportable in a format a caller can inspect (structured, not just log lines).

Acceptance criteria

• A session's trace shows every navigation/request/action in order with timestamps
• Trace overhead is measured and small enough to leave on by default, or is opt-in with the cost stated
• Device test confirms trace contents match a scripted sequence of actions