fix(chat): keep thinking <details> open while streaming

ChatView keyed the transcript each-block by object identity, but core/chunks
returns new RenderedChunk objects per delta, so Svelte recreated each
<article>/<details> every frame — an opened Thinking element snapped shut on
the next token. Key by stable identity instead (c${seq} for committed, p${i}
for append-only provisional) so streaming reuses the DOM. Adds a regression
test that the <details> stays open across a streaming update.

Verified: svelte-check 0/0, vitest 222, biome clean, build ok.

2 files changed, 35 insertions, 1 deletions

diff --git a/src/features/chat/ui.test.ts b/src/features/chat/ui.test.ts
index c4793a0..aebb97c 100644
--- a/src/features/chat/ui.test.ts
+++ b/src/features/chat/ui.test.ts
@@ -155,6 +155,40 @@ describe("ChatView", () => {
 		expect(log).toBeInTheDocument();
 		expect(log.children).toHaveLength(0);
 	});
+
+	it("thinking <details> stays open across a streaming update", async () => {
+		const initial: RenderedChunk[] = [
+			{
+				seq: null,
+				role: "assistant",
+				chunk: { type: "thinking", text: "Let me think..." },
+				provisional: true,
+			},
+		];
+
+		const { rerender } = render(ChatView, { props: { chunks: initial } });
+
+		const details = screen.getByText("Thinking").closest("details");
+		expect(details).not.toBeNull();
+		expect(details).not.toHaveAttribute("open");
+		if (details) details.open = true;
+		expect(details).toHaveAttribute("open");
+
+		const updated: RenderedChunk[] = [
+			{
+				seq: null,
+				role: "assistant",
+				chunk: { type: "thinking", text: "Let me think... step by step" },
+				provisional: true,
+			},
+		];
+		await rerender({ chunks: updated });
+
+		const detailsAfter = screen.getByText("Thinking").closest("details");
+		expect(detailsAfter).not.toBeNull();
+		expect(detailsAfter).toHaveAttribute("open");
+		expect(detailsAfter).toHaveTextContent("Let me think... step by step");
+	});
 });
 
 describe("Composer", () => {
diff --git a/src/features/chat/ui/ChatView.svelte b/src/features/chat/ui/ChatView.svelte
index a7c39cc..ce66798 100644
--- a/src/features/chat/ui/ChatView.svelte
+++ b/src/features/chat/ui/ChatView.svelte
@@ -5,7 +5,7 @@
 </script>
 
 <div class="chat-transcript" role="log" aria-live="polite">
-	{#each chunks as rendered (rendered)}
+	{#each chunks as rendered, i (rendered.seq != null ? `c${rendered.seq}` : `p${i}`)}
 		<article
 			class="message message--{rendered.role}"
 			class:message--provisional={rendered.provisional}