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).

3 files changed, 12 insertions, 0 deletions

diff --git a/packages/kernel/src/contracts/events.ts b/packages/kernel/src/contracts/events.ts
index 8737b02..be09066 100644
--- a/packages/kernel/src/contracts/events.ts
+++ b/packages/kernel/src/contracts/events.ts
@@ -13,6 +13,7 @@ export type {
 	TurnReasoningDeltaEvent,
 	TurnSealedEvent,
 	TurnStartEvent,
+	TurnStepCompleteEvent,
 	TurnTextDeltaEvent,
 	TurnToolCallEvent,
 	TurnToolOutputEvent,
diff --git a/packages/kernel/src/contracts/index.ts b/packages/kernel/src/contracts/index.ts
index 1698486..38f1442 100644
--- a/packages/kernel/src/contracts/index.ts
+++ b/packages/kernel/src/contracts/index.ts
@@ -35,6 +35,7 @@ export type {
 	TurnReasoningDeltaEvent,
 	TurnSealedEvent,
 	TurnStartEvent,
+	TurnStepCompleteEvent,
 	TurnTextDeltaEvent,
 	TurnToolCallEvent,
 	TurnToolOutputEvent,
diff --git a/packages/kernel/src/contracts/runtime.ts b/packages/kernel/src/contracts/runtime.ts
index 8917709..b7fe23c 100644
--- a/packages/kernel/src/contracts/runtime.ts
+++ b/packages/kernel/src/contracts/runtime.ts
@@ -90,6 +90,16 @@ export interface RunTurnInput {
 	 * emitted (backward-compatible with callers that don't yet pass a logger).
 	 */
 	readonly logger?: Logger;
+
+	/**
+	 * Optional monotonic-ish clock (milliseconds) for emitting wall-clock timing
+	 * on outward events: per-step `step-complete` (ttft/decode/genTotal), tool
+	 * execution `durationMs` on `tool-result`, and turn `durationMs` on `done`.
+	 * Injected (not ambient) so the runtime stays pure and deterministic in tests.
+	 * If omitted, the runtime emits no such timing (the optional fields stay
+	 * absent) — backward-compatible with callers that don't provide a clock.
+	 */
+	readonly now?: () => number;
 }
 
 /**