WritableChip::Save() wrote the image in place. A crash or a kill while
it is being written leaves a truncated file: the Dreamcast then boots
into the date and time setup with its settings gone, and an arcade
board loses its nvmem (service settings, high scores, unlocked content).
Write to a temp file next to the real one, flush it, then rename it
over the real path, so the file on disk is always either the previous
image or the complete new one. On Windows, rename() refuses to replace
an existing file, so flycast::rename() (which had no callers) now uses
MoveFileEx with MOVEFILE_REPLACE_EXISTING, matching POSIX.
Testing: compiles on macOS arm64; the Windows path is not built here.
Co-authored-by: Claude Fable 5.1 <[email protected]>
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