fix(core): serialise attachment file mutations, atomic writes, reject svg

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

packages/core/test/crypto.test.ts

@@ -23,4 +23,17 @@ describe("crypto", () => {
     await expect(decryptJson("k", "garbage")).rejects.toBeInstanceOf(CryptoError);
     await expect(decryptJson("k", "v2.AA==.AA==")).rejects.toBeInstanceOf(CryptoError);
   });
+
+  test("IV decodes to 12 bytes", async () => {
+    const blob = await encryptJson("k", { a: 1 });
+    expect(Buffer.from(blob.split(".")[1]!, "base64")).toHaveLength(12);
+  });
+
+  test("flipping a ciphertext byte throws CryptoError", async () => {
+    const [v, iv, ct] = (await encryptJson("k", { a: 1 })).split(".");
+    const bytes = Buffer.from(ct!, "base64");
+    bytes[0] = bytes[0]! ^ 0xff;
+    const tampered = `${v}.${iv}.${bytes.toString("base64")}`;
+    await expect(decryptJson("k", tampered)).rejects.toBeInstanceOf(CryptoError);
+  });
 });