FS25 KW W900 repair

- Added nil guards to LightExtension.lua onUpdate/onDelete.
- Corrected modDesc icon and MoreDesignConfigs paths.
- Removed missing W900L.xml store item.
- Removed invalid second node from workingLight26.xml.
- Removed unresolved legacy sound-template entries.
- Replaced missing hydrostaticTransmission.wav reference.
- Set beltActivationTrigger collision group to bit 29.

Original archive was not modified.

V3 cleanup:
- Converted sounds/TrainHorn.ogg to mono for FS25 3D positional audio.
- Added exactFillRootNode to diesel fill units to address fuel fill-unit warning.
- Removed the defaultLight entry currently reported by FS25 as defaultLight(20) for invalid light shader parameters.
- Hidden shared-light warnings were left untouched to avoid deleting optional physical work-light configurations.

V4: Removed four obsolete combination references; avoided duplicate design9-design16 registration by starting custom registration at design17; moved fuel exactFillRootNode to a known valid visible vehicle node; removed six hidden workingLight26 shared-light links that generated repeated warnings.
