feat(wire,kernel,session-orchestrator): live turn metrics on the stream

Expose the backend's authoritative token+timing metrics on the live AgentEvent
stream (observability-only -> now also client-facing). All additive/optional.

- [email protected]: new TurnStepCompleteEvent (type:step-complete) with per-step
  ttftMs/decodeMs/genTotalMs; usage += stepId; tool-result += durationMs (exec);
  done += durationMs (turn wall-clock) + usage (turn total). RunTurnInput += now?.
  [email protected] (re-export bump).
- kernel-runtime: when now injected, measures + emits the above (reuses the
  ttft/decode first-token detection); omits timing gracefully without a clock.
- session-orchestrator: adds now? to deps, threads into RunTurnInput; extension
  activate injects () => Date.now().
- transport/cli/host-bin: untouched (verbatim pass-through; additive fields).

FE handoff: frontend-metrics-handoff.md. typecheck clean; 520 vitest + 89 bun;
biome 0/0. Replay/persistence = deferred Pass 2 (documented in tasks.md).

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