Add model bundle export (Cast + textures) for DS2

[UncleRito]

Summary

When you export a model as Cast today, you get the geometry and per-material mesh pieces — but the actual textures live behind ~10 ref hops the user has to walk by hand for every model. This change adds two new export formats — Cast + DDS textures and Cast + PNG textures — that ship the Cast file alongside every TextureSet transitively reachable from the source object, preserving the surviving directory tree from TextureSetTextureDesc.path:

<export-root>/<ObjectType_g_i>/
  model.cast
  ds/models/characters/fgl_fragile/fgl_textures/textures/
    fgl_body_boots_v00_clr.dds
    fgl_face_fuzzdeferred_v00_clr.dds   ← variation textures included
    ...

Variation TextureSets (e.g. muddy face via ArtPartsVariationReplaceTextureSetResource) come along automatically because they're reachable via outgoing refs.

Design notes

• No duplicated serialization. The bundle exporters delegate to the existing single-file Cast / DDS / PNG exporters by id via the Exporter SPI. AbstractModelBundleExporter is the shared orchestration; concrete subclasses only declare which texture exporter id to use and which extension to emit.
• Lazy texture walk. ModelBundle holds a memoized Supplier<List<TextureSet>> rather than an eager list. ModelToModelBundleConverter.convert() only does the Scene conversion (delegating to the existing converter) and packages the supplier — the BFS runs only when bundle.textureSets() is called inside the exporter. This matters because anything that probes converters eagerly (action visibility checks, viewer discovery, etc.) shouldn't pay the walk cost.
• DS2-only. HFW lacks ArtPartsDataResource and its model containers differ enough that a separate implementation is warranted later. The intermediate ModelBundle type lives in odradek-game so a future HFW converter can reuse it.
• Dedup. Same source-texture path is written at most once per bundle.
• No material binding in Cast. Cast supports it; the existing exporter writes geometry only. Out of scope here — the per-material mesh split means manual assignment in Blender is one selection per material, which is bearable.

Test plan

• ./mvnw clean package succeeds against JDK 25.
• Manual: export an ArtPartsDataResource as "Cast + DDS textures" produces model.cast plus the expected ds/models/... tree.
• Manual: variation TextureSets are bundled (verified by checking that a character with body-state variants gets multiple albedo maps).
• Manual: app remains responsive when opening model editors — the walk is deferred.

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