Slice 2 wave 2: IndexedDB cache adapter + chat feature

- adapters/idb: createIdbChunkStore implements the ConversationChunkStore port
  over IndexedDB (compound [conversationId,seq] key, idempotent append, meta
  store for lastAccess); 8 tests with fake-indexeddb
- features/chat: createChatStore (runes-thin over the core/chunks reducer, all
  effects injected via ChatTransport/HistorySync/ConversationCache ports) +
  ChatView/Composer svelte-thin UI; folds chat.delta, syncs on turn-sealed,
  hydrates from cache then catches up; 25 tests

Verified green: svelte-check 0/0, vitest 202, biome clean, build ok.

2 files changed, 78 insertions, 0 deletions

diff --git a/src/features/chat/ui/ChatView.svelte b/src/features/chat/ui/ChatView.svelte
new file mode 100644
index 0000000..a7c39cc
--- /dev/null
+++ b/src/features/chat/ui/ChatView.svelte
@@ -0,0 +1,45 @@
+<script lang="ts">
+	import type { RenderedChunk } from "../index";
+
+	let { chunks }: { chunks: readonly RenderedChunk[] } = $props();
+</script>
+
+<div class="chat-transcript" role="log" aria-live="polite">
+	{#each chunks as rendered (rendered)}
+		<article
+			class="message message--{rendered.role}"
+			class:message--provisional={rendered.provisional}
+		>
+			<header class="message__role">{rendered.role}</header>
+			<div class="message__content">
+				{#if rendered.chunk.type === "text"}
+					<p>{rendered.chunk.text}</p>
+				{:else if rendered.chunk.type === "thinking"}
+					<details>
+						<summary>Thinking</summary>
+						<p>{rendered.chunk.text}</p>
+					</details>
+				{:else if rendered.chunk.type === "tool-call"}
+					<div class="tool-call">
+						<strong>{rendered.chunk.toolName}</strong>
+						<pre>{JSON.stringify(rendered.chunk.input, null, 2)}</pre>
+					</div>
+				{:else if rendered.chunk.type === "tool-result"}
+					<div class="tool-result" class:tool-result--error={rendered.chunk.isError}>
+						<strong>{rendered.chunk.toolName}</strong>
+						<pre>{rendered.chunk.content}</pre>
+					</div>
+				{:else if rendered.chunk.type === "error"}
+					<div class="error" role="alert">
+						{rendered.chunk.message}
+						{#if rendered.chunk.code}
+							<span class="error__code">[{rendered.chunk.code}]</span>
+						{/if}
+					</div>
+				{:else if rendered.chunk.type === "system"}
+					<div class="system">{rendered.chunk.text}</div>
+				{/if}
+			</div>
+		</article>
+	{/each}
+</div>
diff --git a/src/features/chat/ui/Composer.svelte b/src/features/chat/ui/Composer.svelte
new file mode 100644
index 0000000..dc71e11
--- /dev/null
+++ b/src/features/chat/ui/Composer.svelte
@@ -0,0 +1,33 @@
+<script lang="ts">
+	let { onSend }: { onSend: (text: string) => void } = $props();
+
+	let text = $state("");
+
+	function handleSubmit(): void {
+		const trimmed = text.trim();
+		if (trimmed.length === 0) return;
+		onSend(trimmed);
+		text = "";
+	}
+
+	function handleKeydown(e: KeyboardEvent): void {
+		if (e.key === "Enter" && !e.shiftKey) {
+			e.preventDefault();
+			handleSubmit();
+		}
+	}
+</script>
+
+<form class="composer" onsubmit={prevent => { prevent.preventDefault(); handleSubmit(); }}>
+	<textarea
+		class="composer__input"
+		bind:value={text}
+		onkeydown={handleKeydown}
+		placeholder="Type a message..."
+		rows="3"
+		aria-label="Message input"
+	></textarea>
+	<button class="composer__send" type="submit" disabled={text.trim().length === 0}>
+		Send
+	</button>
+</form>