Savers now serialize to in-memory HISTORY_FILE_DATA on the UI thread,
then Prettify + file writes + git stage/diff/commit run on a background
thread via GetKiCadThreadPool(). An atomic skip-if-busy flag prevents
overlapping saves; WaitForPendingSave() ensures clean shutdown.
Extracted FormatBoardToFormatter and FormatSchematicToFormatter so
SaveToHistory can serialize without touching disk. PROJECT savers use
sourcePath for file-copy semantics since project files are small.
Seth Hillbrand
902048a172