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.

1 files changed, 6 insertions, 0 deletions

diff --git a/src/features/chat/index.ts b/src/features/chat/index.ts
new file mode 100644
index 0000000..71851de
--- /dev/null
+++ b/src/features/chat/index.ts
@@ -0,0 +1,6 @@
+export type { RenderedChunk } from "../../core/chunks";
+export type { ChatTransport, HistorySync } from "./ports";
+export type { ChatStore, ChatStoreDependencies } from "./store.svelte";
+export { createChatStore } from "./store.svelte";
+export { default as ChatView } from "./ui/ChatView.svelte";
+export { default as Composer } from "./ui/Composer.svelte";