React Native incompatible with Xcode 26

[leotm]

• Xcode 26.4b1/b2/b3/RC • FMT Compile Errors (it needs upgrading) • REPRO **IS** included despite the erroneous label react/react-native#55601
• Bump fmt to 12.1.0 to fix xcode 26.4 react/react-native#56099 fixed earlier upstream
  • [0.83] Fix XCode 26.4 build errors reactwg/react-native-releases#1259 backported
  • [0.81] Fix XCode 26.4 build errors reactwg/react-native-releases#1261
  • [0.82] Fix XCode 26.4 build errors reactwg/react-native-releases#1262
  • [0.84] Fix XCode 26.4 build errors reactwg/react-native-releases#1263

[leotm]

bumped in prev nightly including upstream fix