Slice 2 wave 1: transcript reducer, wire conformance, ws chat, cache core

- core/chunks: the one pure transcript reducer (foldEvent live deltas +
  applyHistory seq-keyed reconcile + selectChunks/selectMessages); 27 tests
- core/wire: FE-side contract-conformance exhaustiveness guards + drift smoke
  tests over wire/transport-contract unions (§2.9 drift signal); 10 tests
- adapters/ws: additively multiplex chat.send/chat.delta/chat.error on the
  existing surface socket (onChat + widened send); surface API unchanged
- features/conversation-cache: pure reconcileCache/nextSinceSeq/selectEvictions
  + ConversationChunkStore port + injected createConversationCache; 26 tests

Verified green: svelte-check 0/0, vitest 169, biome clean, build ok.

1 files changed, 42 insertions, 0 deletions

diff --git a/src/features/conversation-cache/types.ts b/src/features/conversation-cache/types.ts
new file mode 100644
index 0000000..2a349cc
--- /dev/null
+++ b/src/features/conversation-cache/types.ts
@@ -0,0 +1,42 @@
+import type { StoredChunk } from "@dispatch/wire";
+
+/** Metadata entry for a cached conversation, used by eviction logic. */
+export interface ConversationCacheIndexEntry {
+	readonly conversationId: string;
+	readonly chunkCount: number;
+	readonly maxSeq: number;
+	readonly lastAccess?: number;
+}
+
+/**
+ * Storage port for conversation chunk persistence.
+ *
+ * The IndexedDB implementation lives in `src/adapters/idb/` (separate unit);
+ * this interface is the contract the cache logic depends on.
+ *
+ * All methods MUST be idempotent on `seq`: re-appending an existing seq is a no-op.
+ */
+export interface ConversationChunkStore {
+	/** Load all cached chunks for a conversation, seq-ordered. Returns [] if absent. */
+	load(conversationId: string): Promise<readonly StoredChunk[]>;
+
+	/**
+	 * Append committed chunks to a conversation's cache.
+	 * MUST be idempotent on `seq`: re-appending an existing seq is a no-op.
+	 */
+	append(conversationId: string, chunks: readonly StoredChunk[]): Promise<void>;
+
+	/** Delete all cached data for a conversation. */
+	delete(conversationId: string): Promise<void>;
+
+	/** Return metadata for all cached conversations (for eviction). */
+	index(): Promise<readonly ConversationCacheIndexEntry[]>;
+}
+
+/** Result of reconciling cached chunks with incoming authoritative chunks. */
+export interface ReconcileResult {
+	/** The merged, deduplicated, seq-ordered chunk list. */
+	readonly merged: readonly StoredChunk[];
+	/** The subset of incoming chunks that need to be appended (not already cached). */
+	readonly toAppend: readonly StoredChunk[];
+}