Add SecureEnclave stub

### New API Proposal: SecureEnclave

#### Motivation:

Swift packages that use [CryptoKit's SecureEnclave module](https://developer.apple.com/documentation/cryptokit/secureenclave) (if it is available) cannot compile against Crypto, since Crypto does not define this module. All SecureEnclave operations have to be conditionally compiled in this case.

Since SecureEnclave has runtime detection (`SecureEnclave.isAvailable`), a stub of the SecureEnclave API that returns `false` for isAvailable (and throws for all operations) on non-macOS platforms would remove the need for conditional compilation.

One annoyance is that some functions in this API also take `LAContext` and `SecAccessControl` parameters. These can be left out, or the structures would have to be stubbed out too.

#### Importance:

Better compatibility with CryptoKit.

Start by reviewing CryptoKit's SecureEnclave API from the linked documentation and comparing it with the repository's existing cross-platform API structure. Define the supported stub surface, including whether LAContext and SecAccessControl are included. Done means non-macOS builds can import the module, isAvailable is false, and supported operations throw as specified.

Written by the indexing model from the issue text.