perf(chat): ingest a message batch in one transaction

Ingesting a scan's worth of chat messages persisted each new message
with its own append() call, so a batch of fresh messages opened and
committed one SQLite transaction per row.

- A new appendBatch persists many messages in a single transaction,
  using the same INSERT OR IGNORE dedup key as append() and preserving
  input order so load()'s rowid ordering is unchanged.
- Every message is serialized and encrypted before the transaction
  opens, so a message with a non-UTF-8 body is skipped on its own
  without leaving a transaction open; the valid siblings still persist.
- Ingest collects the newly stored messages and writes them once after
  the loop instead of per message. It is inert (returns 0) while locked
  or on empty input.

Tests cover a batch insert, dedup on re-insert, empty input, bad-UTF-8
isolation with a valid sibling and a following append(), insert order,
and the locked no-op.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_phase4.cpp

@@ -14274,6 +14274,34 @@ void testHushChatDatabase()
         EXPECT_TRUE(loaded[0].delivery == ChatDelivery::Failed);
     }
 
+    // appendBatch: one transaction, dedup, order, tombstone-skip, bad-UTF-8 isolation.
+    scrub(dbPath);
+    {
+        ChatDatabase db(dbPath);
+        EXPECT_TRUE(db.unlockWithSecret(seedA));
+        std::vector<ChatMessage> batch = {
+            makeMsg("b1", 0, "c", "one",   ChatMessageKind::Message, 1),
+            makeMsg("b2", 0, "c", "two",   ChatMessageKind::Message, 2),
+            makeMsg("b3", 0, "c", "three", ChatMessageKind::Message, 3),
+        };
+        EXPECT_EQ(db.appendBatch(batch), 3);
+        EXPECT_EQ(db.appendBatch(batch), 0);                 // dedup: all ignored
+        EXPECT_EQ((int)db.appendBatch({}), 0);               // empty → no-op
+        // bad UTF-8 body is skipped, the valid sibling still persists, and append() still works after.
+        ChatMessage bad = makeMsg("b4", 0, "c", std::string("\xff\xfe", 2), ChatMessageKind::Message, 4);
+        ChatMessage good = makeMsg("b5", 0, "c", "five", ChatMessageKind::Message, 5);
+        EXPECT_EQ(db.appendBatch({bad, good}), 1);           // only good persisted, no open txn left
+        EXPECT_TRUE(db.append(makeMsg("b6", 0, "c", "six", ChatMessageKind::Message, 6)));
+        std::vector<ChatMessage> all = db.load();
+        EXPECT_EQ((int)all.size(), 5);                       // b1,b2,b3,b5,b6
+        EXPECT_EQ(all[0].body, std::string("one"));          // rowid/insert order
+        EXPECT_EQ(all[4].body, std::string("six"));
+    }
+    {
+        ChatDatabase locked(dbPath);                         // no unlock → inert
+        EXPECT_EQ(locked.appendBatch({makeMsg("x", 0, "c", "x", ChatMessageKind::Message, 0)}), 0);
+    }
+
     scrub(dbPath); scrub(dbPath2);
 }