Restructure into src/ layout under pygui package

Move all application source under src/pygui/ and rewire imports,
build config, and QML module path to match.
- Relocate backend/, serialcomm/, and the ISC QML module into
  src/pygui/; convert main.py into pygui/__main__.py with a main()
  entry point (run via `python -m pygui` or the new `isc` script)
- Rewrite absolute imports: backend.* -> pygui.backend.*,
  serialcomm.* -> pygui.serialcomm.* (source + tests)
- Move app icons (isc.ico/icns) into packaging/
- Update README and ISC.qmlproject to the new paths

src/pygui/backend/crypto_helper.py

@@ -0,0 +1,90 @@
+import secrets
+from pathlib import Path
+from typing import Union
+
+from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
+from cryptography.hazmat.backends import default_backend
+
+
+# ===== Crypto Utilities =====
+class CryptoHelper:
+    """Cryptographic utilities for wafer data encryption/decryption."""
+
+    # ===== Hex/Byte Conversion =====
+    @staticmethod
+    def hex_string_to_bytearray(hex_str: str) -> bytes:
+        """Converts a hex string to bytes (e.g., '48656C6C6F' → b'Hello')."""
+        if len(hex_str) % 2 != 0:
+            raise ValueError("Hex string must have even length")
+        try:
+            return bytes.fromhex(hex_str)
+        except ValueError as e:
+            raise ValueError(f"Invalid hex string: {e}")
+
+    @staticmethod
+    def bytearray_to_hex_string(data: Union[bytes, bytearray]) -> str:
+        """Converts bytes to lowercase hex string."""
+        return data.hex()
+
+    # ===== Random Data =====
+    @staticmethod
+    def generate_random_bytes(length: int) -> bytes:
+        """Generates cryptographically secure random bytes."""
+        if length < 0:
+            raise ValueError("Length must be non-negative")
+        return secrets.token_bytes(length)
+
+    # ===== AES Decryption =====
+    @staticmethod
+    def decrypt_aes(data: bytes, key: bytes, iv: bytes) -> bytes:
+        """
+        Decrypts AES-128-CBC data with PKCS7 padding.
+
+        Args:
+            data: Encrypted ciphertext (must be multiple of 16 bytes)
+            key: 16-byte AES key
+            iv: 16-byte IV
+
+        Returns:
+            Decrypted plaintext
+
+        Raises:
+            ValueError: If inputs are invalid
+            cryptography.exceptions.InvalidSignature: On decryption failure
+        """
+        if len(key) != 16:
+            raise ValueError("Key must be 16 bytes (AES-128)")
+        if len(iv) != 16:
+            raise ValueError("IV must be 16 bytes")
+        if len(data) == 0:
+            return b""
+        if len(data) % 16 != 0:
+            raise ValueError("Ciphertext length must be multiple of 16 bytes")
+
+        cipher = Cipher(algorithms.AES(key), modes.CBC(iv), backend=default_backend())
+        decrypt_ctx = cipher.decryptor()
+        plaintext_padded = decrypt_ctx.update(data) + decrypt_ctx.finalize()
+
+        # Remove PKCS7 padding
+        pad_len = plaintext_padded[-1]
+        if pad_len > 16 or pad_len == 0:
+            raise ValueError("Invalid padding")
+        return plaintext_padded[:-pad_len]
+
+    # ===== Binary File I/O =====
+    @staticmethod
+    def save_to_binary_file(file_path: Union[str, Path], data: bytes) -> None:
+        """Writes bytes to a binary file."""
+        Path(file_path).write_bytes(data)
+
+    @staticmethod
+    def read_from_binary_file(file_path: Union[str, Path]) -> bytes:
+        """Reads bytes from a binary file."""
+        return Path(file_path).read_bytes()
+
+    # ===== Convenience Helpers =====
+    @classmethod
+    def read_hex_string_from_binary_file(cls, file_path: Union[str, Path]) -> str:
+        """Reads binary file and returns hex string."""
+        data = cls.read_from_binary_file(file_path)
+        return cls.bytearray_to_hex_string(data)