v0.5.0 - cargo variants

Install: download simutrans_blender_kit.zip below, then in Blender go to Edit → Preferences → Add-ons → Install…, pick the zip, and enable "Simutrans Blender Kit".

Added

• Cargo variants (freight images). A wagon can now show a different sprite for each good it carries — empty, loaded with coal, loaded with oil. Put each load in a collection freight_0, freight_1, … (the same additive convention as seasons), list the goods in the new Cargo variants field (Kohle, Oel), and the render produces the empty sheet plus one loaded sheet per good and writes the emptyimage / freightimage[i] / freightimagetype[i] .dat the engine needs.

  Grounded in the engine source throughout: the engine requires exactly one freightimagetype per freight image and makeobj FATALs without it; it shows the empty image when the wagon is unloaded and the matching freight image when it is not, falling back to index 0. There are no "livery" images in base Simutrans — that is a Simutrans Extended feature — so this is freight, precisely.

  Verified end to end: the emitted .dat compiles under makeobj (and fails without the freightimagetype lines); the empty and loaded sprites differ pixel for pixel in every heading; and the wagon loads in the running engine, is buyable, and its freight goods really resolve — which only the game can prove, because makeobj defers the goods to load time.

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Everything from v0.4.0 (render progress + cancel, CI) is included.