feat(turns): detached turns + multi-client live view

A turn no longer dies when its WebSocket connection closes. The turn-broadcast
hub moves into the core (session-orchestrator): turns run detached, persist at
seal regardless of clients, and fan out AgentEvents to N subscribers per
conversation with in-flight buffer replay for late-joiners. transport-ws stops
aborting turns on socket close and gains chat.subscribe/chat.unsubscribe so a
second device (or a reloaded browser) can watch a running turn.

- @dispatch/transport-contract 0.6.0->0.7.0: chat.subscribe/chat.unsubscribe WS ops
- session-orchestrator: startTurn/subscribe/isActive; persistent subscribers +
  per-turn buffer (two-map model); handleMessage = convenience wrapper (no signal)
- transport-ws: per-connection chat-subscription fan-out; no turn-abort-on-close
- transport-http: test fakes updated for the widened interface (runtime unchanged)
- design notes/turn-continuity-design.md; FE courier frontend-turn-continuity-handoff.md

Live-verified vs flash (2-client WS): sender disconnect mid-turn -> other client
streams to done + turn persists; late-join replays turn from turn-start. 891 vitest
+ transport bun green; tsc -b EXIT 0; biome clean.

2 files changed, 36 insertions, 0 deletions

diff --git a/packages/transport-http/src/app.test.ts b/packages/transport-http/src/app.test.ts
index 07f6777..32e9689 100644
--- a/packages/transport-http/src/app.test.ts
+++ b/packages/transport-http/src/app.test.ts
@@ -106,6 +106,15 @@ function createFakeConversationStore(
 
 function createFakeOrchestrator(events: AgentEvent[]): SessionOrchestrator {
 	return {
+		startTurn() {
+			return { started: true, turnId: "fake-turn" };
+		},
+		subscribe() {
+			return () => {};
+		},
+		isActive() {
+			return false;
+		},
 		async handleMessage(input) {
 			for (const event of events) {
 				input.onEvent(event);
@@ -124,6 +133,15 @@ function createCapturingOrchestrator(): SessionOrchestrator & {
 		get received() {
 			return state.received;
 		},
+		startTurn() {
+			return { started: true, turnId: "fake-turn" };
+		},
+		subscribe() {
+			return () => {};
+		},
+		isActive() {
+			return false;
+		},
 		async handleMessage(input) {
 			state.received = input;
 		},
@@ -132,6 +150,15 @@ function createCapturingOrchestrator(): SessionOrchestrator & {
 
 function createThrowingOrchestrator(error: Error): SessionOrchestrator {
 	return {
+		startTurn() {
+			return { started: true, turnId: "fake-turn" };
+		},
+		subscribe() {
+			return () => {};
+		},
+		isActive() {
+			return false;
+		},
 		async handleMessage() {
 			throw error;
 		},
diff --git a/packages/transport-http/src/server.bun.test.ts b/packages/transport-http/src/server.bun.test.ts
index b43469f..8a719c0 100644
--- a/packages/transport-http/src/server.bun.test.ts
+++ b/packages/transport-http/src/server.bun.test.ts
@@ -55,6 +55,15 @@ function fakeConversationStore(): ConversationStore {
 
 function fakeOrchestrator(): SessionOrchestrator {
 	return {
+		startTurn() {
+			return { started: true, turnId: "fake-turn" };
+		},
+		subscribe() {
+			return () => {};
+		},
+		isActive() {
+			return false;
+		},
 		async handleMessage() {},
 	};
 }