The engine already implements prime_volume_mode (Default/Saving/Fast)
but nothing in the UI could set it, leaving prime-saving unreachable on
multi-sub-nozzle extruders and fast purge unreachable on printers that
support it.
- new PurgeModeDialog with selectable Standard/Fast or
Standard/Prime Saving cards depending on printer capability
- "Purge mode" sidebar button next to Flushing volumes; opens the
dialog and stores the choice in the project config
- printer preset-load gating: button shown only when the printer has
multiple sub-nozzles per extruder or sets support_fast_purge_mode;
stale project values the printer cannot honor reset to Default
- enable fast purge on A2L 0.4 (support_fast_purge_mode), explicit
default 0 in the common machine base
- new dialog strings added to OrcaSlicer.pot
Printers without these capabilities never show the button and their
projects keep prime_volume_mode at Default, so slicing output is
unchanged.
