Mirror DISPLAY fallback in native spawns

[emil-e]

Summary

• build native process environments in reverse process-environment order so first entries and bare unsets match Emacs semantics
• synthesize DISPLAY for native spawns when graphical Emacs has a DISPLAY fallback and no explicit DISPLAY entry exists
• add native regression coverage for DISPLAY fallback, non-graphical frames, bare unsets, and duplicate env precedence

Test

• make .build/tests/native-ghostel-native-process-test.ok