feat(metrics): per-turn + per-step token/timing metrics bubbles

Consume [email protected] / [email protected] metrics: usage.stepId,
step-complete (ttft/decode/genTotal), done.durationMs/usage, and the
durable GET /conversations/:id/metrics endpoint.

- core/metrics: pure live-fold + durable-merge reducer; decode-rate TPS;
  head-aligned, stable placement; progressive per-step rows (each shown as
  its step ends) with the turn-total row gated on the done event.
- features/chat: store folds metric events + hydrates durable TurnMetrics;
  ChatView renders inline step bubbles + a turn-total bubble.
- app: MetricsSync HTTP effect (tolerates 404) injected into chat stores.
- scripts/live-probe: drives the metrics path; live-verified 17/17 vs bin/up.
- docs: regenerate .dispatch wire/transport mirrors to 0.4.0; glossary terms
  (turn/step metrics, TTFT, decode time, TPS, metrics bubble); trim handoff.

2 files changed, 14 insertions, 2 deletions

diff --git a/src/app/App.svelte b/src/app/App.svelte
index 61b4cb9..857a1e5 100644
--- a/src/app/App.svelte
+++ b/src/app/App.svelte
@@ -62,7 +62,7 @@
 
 		<div class="flex-1 overflow-y-auto">
 			{#key store.activeConversationId}
-				<ChatView chunks={store.activeChat.chunks} />
+				<ChatView chunks={store.activeChat.chunks} turnMetrics={store.activeChat.turnMetrics} />
 			{/key}
 		</div>
 
diff --git a/src/app/store.svelte.ts b/src/app/store.svelte.ts
index 760c390..fe3c55c 100644
--- a/src/app/store.svelte.ts
+++ b/src/app/store.svelte.ts
@@ -2,6 +2,7 @@ import type {
 	ChatDeltaMessage,
 	ChatErrorMessage,
 	ConversationHistoryResponse,
+	ConversationMetricsResponse,
 	ModelsResponse,
 } from "@dispatch/transport-contract";
 import type { SurfaceServerMessage, SurfaceSpec } from "@dispatch/ui-contract";
@@ -17,7 +18,7 @@ import {
 	subscribe as protocolSubscribe,
 	unsubscribe as protocolUnsubscribe,
 } from "../core/protocol";
-import type { ChatStore } from "../features/chat";
+import type { ChatStore, MetricsSync } from "../features/chat";
 import { createChatStore } from "../features/chat";
 import type { ConversationCache } from "../features/conversation-cache";
 import { createConversationCache } from "../features/conversation-cache";
@@ -73,6 +74,15 @@ function createHistorySync(
 	};
 }
 
+function createMetricsSync(httpBase: string, fetchImpl: typeof fetch): MetricsSync {
+	return async (conversationId: string) => {
+		const url = `${httpBase}/conversations/${encodeURIComponent(conversationId)}/metrics`;
+		const res = await fetchImpl(url);
+		if (!res.ok) return { turns: [] };
+		return (await res.json()) as ConversationMetricsResponse;
+	};
+}
+
 export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
 	let protocol = $state<ProtocolState>(protocolInitialState());
 	let selectedId = $state<string | null>(null);
@@ -112,6 +122,7 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
 	);
 
 	const historySync = createHistorySync(httpBase, fetchImpl);
+	const metricsSync = createMetricsSync(httpBase, fetchImpl);
 
 	const chatStores = new Map<string, ChatStore>();
 
@@ -125,6 +136,7 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
 				},
 			},
 			historySync,
+			metricsSync,
 			cache,
 		});
 	}