Delete the keychain item or Secure Enclave key of a failed unlocker add (closes #89)

A Secure Enclave unlocker add gets the long-term key before it creates
the Secure Enclave key, so a wrong passphrase creates none, and deletes
the key if a later step fails. macse.CreateKey finds the new key's hash
right after sc_auth creates it, failing with an error naming the label
if it cannot, and deletes the key if getting its public key then fails.
A keychain unlocker add writes all of the unlocker's files before it
stores the keychain item, and deletes the item if moving the unlocker
into place then fails. A failure to delete is reported along with the
original error.

The Objective-C and macse_darwin.go were only read, never compiled or
run; the new tests run only on a Mac.

Model: opus-5-5

internal/macse/secure_enclave.h

@@ -5,20 +5,30 @@
 
 #include <stdint.h>
 
-// se_create_key creates a new P-256 key in the Secure Enclave via sc_auth.
+// se_create_key creates a new P-256 key in the Secure Enclave via sc_auth and
+// finds its identity hash. If the hash cannot be found, the key exists but
+// se_create_key fails, with an error naming the label.
 // label: unique identifier for the CTK identity (UTF-8 C string)
-// pub_key_out: output buffer for the uncompressed public key (65 bytes for P-256)
-// pub_key_len: on input, size of pub_key_out; on output, actual size written
 // hash_out: output buffer for the identity hash (for deletion)
 // hash_out_len: size of hash_out buffer
 // error_out: output buffer for error message
 // error_out_len: size of error_out buffer
 // Returns 0 on success, -1 on failure.
 int se_create_key(const char *label,
-                  uint8_t *pub_key_out, int *pub_key_len,
                   char *hash_out, int hash_out_len,
                   char *error_out, int error_out_len);
 
+// se_copy_public_key copies the public key of a CTK identity.
+// label: label of the CTK identity
+// pub_key_out: output buffer for the uncompressed public key (65 bytes for P-256)
+// pub_key_len: on input, size of pub_key_out; on output, actual size written
+// error_out: output buffer for error message
+// error_out_len: size of error_out buffer
+// Returns 0 on success, -1 on failure.
+int se_copy_public_key(const char *label,
+                       uint8_t *pub_key_out, int *pub_key_len,
+                       char *error_out, int error_out_len);
+
 // se_encrypt encrypts data using the SE-backed public key (ECIES).
 // label: label of the CTK identity whose public key to use
 // plaintext: data to encrypt