fix(d5-daemon-auth): address auth review findings (Wave B)

- test_credentials.c: NUL-terminate the overlong-line stack buffer before
  make_tmp_file's strlen() (was a stack-buffer-overflow READ under ASan);
  still exercises the overlong-rejection path.
- Add redacted protocol string variants (protocol_send_str_redacted /
  receive + fd send_str_redacted/receive_str_redacted) and use them for the
  daemon auth username/digest so --verbose / LOG_DEBUG_ALL never logs a
  replayable credential while other protocol strings keep their debug trace.
- credentials_verify/gate: replace byte-wise-short-circuiting strcmp with a
  fixed-length constant-time username compare (closes user-enumeration oracle);
  update doc comment to match.
- read_secret_file: preserve password exact bytes (only strip trailing CR/LF)
  and burn the stack line buffer; document the whitespace behavior.
- test_server_cli.c: note the parser zero-inits opts on failure.
- Add debug-level daemon test asserting the digest never appears under --verbose.

PROTOCOL_VERSION stays 2.15.0.

src/shared/config.c

@@ -1110,7 +1110,10 @@ static bool send_daemon_auth(int fd, const Config* c) {
     return false;
   if (!present)
     return true;
-  return send_str(fd, c->auth_user) && send_str(fd, c->auth_password_hash);
+  /* Redacted send: the username and hard-wired digest must never reach a
+   * --verbose debug log (they are replayable), while normal protocol strings
+   * keep their debug trace. */
+  return send_str_redacted(fd, c->auth_user) && send_str_redacted(fd, c->auth_password_hash);
 }
 
 static bool receive_daemon_auth(int fd, Config* c) {
@@ -1119,8 +1122,9 @@ static bool receive_daemon_auth(int fd, Config* c) {
     return false;
   if (!present)
     return true;
-  char* user = receive_str(fd);
-  char* hash = receive_str(fd);
+  /* Redacted receive: never log the incoming username/digest bodies. */
+  char* user = receive_str_redacted(fd);
+  char* hash = receive_str_redacted(fd);
   if (!user || !hash) {
     free(user);
     free(hash);