Keep a Disabled Filament Override Off After a Restart
A child preset stores "nil" both for an override the user switched off and
for a per-variant column it inherits from its parent, and the loader filled
every nil from the parent. Whenever the parent preset sets Retract Before
Wipe or another override, switching it off lasted only until the next
restart, project open or newer cloud copy.
Treat the keys in is_filament_extruder_override_key, now including the
ironing overrides, as "nil means off": Preset::save and the project-embedded
preset diff write such an option with all its columns, and the loader keeps
its nil columns while still mapping them onto the parent's variants by
name. Every other nullable option keeps nil as "inherit this column". The
cloud diff only force-emits a disabled override whose default is a real
value.