refactor(sourceengine): extract Source Engine into a plugin + schema-driven exporter contract

[dhkatz]

Summary

Source Engine support now lives in a standalone GMConverter.SourceEngine plugin assembly. Core no longer contains MDL import/export code, the Source/ directory, the MdlCrowbar package reference, or the CoACD MSBuild target. The SDK exporter contract also moves to a schema-driven shape that a future generic UI panel can render directly.

Plugin extraction

• New GMConverter.SourceEngine project (net10.0 library) referencing only GMConverter.SDK + MdlCrowbar + ImageSharp.
• 12 files moved from Core: MDLImporter, MDLExporter, all of Source/* (GameInfo, MaterialOptimizationOptions, PhysicsOptions, SourceMaterialCompiler, SourceMaterialSurfaceProps, SourcePhongSettings, SourceToolPaths), plus the Source-only helpers ProcessRunner, CoacdNative, CoacdDecompositionOptions.
• The plugin's CoACD MSBuild download target moved with it; lib_coacd is resolved at runtime via PluginLoadContext's sibling-folder fallback (introduced in fix(unrealengine): copy NuGet runtime DLLs into plugin bin so deploy ships them #26 for CUE4Parse-Natives).
• SourceEnginePlugin : IPlugin registers MDLImporter and MDLExporter via DI; both receive ITextureFactory via constructor injection.
• plugin.json id: gmconverter.sourceengine.
• Source-specific texture transforms (ToSourcePhongExponent, WithMaskInAlpha) reimplemented in the plugin against the SDK's new Texture.GetRgbaPixels() surface + ITextureFactory.FromRgba(). Core's TextureExtensions keeps the general-purpose transforms (ToGltfMetallicRoughness, WithOpenGlNormalMap, ToSpecularFactorMask).

Schema-driven exporter contract

• New SDK types: OptionType (enum), OptionDescriptor (data, with deferred-default factory hook), OptionGroup, ExporterOptionSchema, ExportOptions (typed-accessor bag).
• IExporter<TOptions> and IExporterDescriptor collapsed into a single non-generic IExporter exposing OptionSchema + Export(Model, string, string, ExportOptions). Plugins read what they need from the bag and assemble their own internal typed options.
• MDLExporter declares the full ~12-option schema across Tools / Materials / Physics groups. GLTFExporter declares a minimal {binary, bakeUvTransforms} schema; OBJExporter declares ExporterOptionSchema.Empty.

Host bundling

• GMConverter.CLI.csproj and GMConverter.UI.csproj both add a <BundledPlugin> entry for SourceEngine (same shape as the existing UnrealEngine entry). CopyBundledPlugins stages the plugin's bin/ into bin/.../plugins/sourceengine/.
• The host's MdlCrowbar package ref is removed from Core's csproj.

Host-side surface cleanup

• CLI Program.cs and UI ConversionService.cs no longer reference plugin-private Source types (PhysicsOptions, PhysicsMode, CoacdOptions, SourceToolPaths, MDLExporter, MDLImporter). Both build ExportOptions bags from primitive inputs and look up the exporter via PluginHost.Registry.GetExporter(\"mdl\"). Failing that, they surface a clear error pointing at the missing plugin.
• ConvertViewModel's auto-fill of StudioMDL / VTFCmd paths uses a small local copy of the path-discovery convention (./tools/<tool>/...) instead of calling into the plugin. The plugin keeps its own copy for the resolve path used at export time.
• UI's CoACD physics preview removed; preview falls back to a bounds visualisation for both modes. Actual export still produces CoACD physics when the user picks that mode. Surfacing a plugin-contributed preview hook is a follow-up.

Empirically verified

• CLI smoke: running against an empty test.psk, plugin log line Loaded plugin gmconverter.sourceengine v0.1.0 appears alongside Loaded plugin gmconverter.unrealengine v0.1.0, importer error surfaces as expected.
• Plugin deploy: bin/Release/net10.0/plugins/sourceengine/ contains GMConverter.SourceEngine.dll, MdlCrowbar.dll, lib_coacd.dll, plugin.json and the rest of the runtime deps.
• dotnet build GMConverter.slnx -c Release → 0 warnings, 0 errors
• dotnet format --verify-no-changes → clean

Deferred to follow-up (explicit non-goals)

• Generic schema-driven UI panel (ExporterOptionsPanel.axaml + DataTemplates + ExporterOptionsViewModel). The existing SourceEngineSettings.axaml panel still binds ConvertViewModel's typed Source properties; ConversionService pipes those into the ExportOptions bag at runtime. The schema is the source of truth for keys and defaults but rendering remains bespoke for Source until the generic panel lands.
• CLI dynamic argument construction from the schema — the schema declares the options, but the CLI still has hand-rolled --studiomdl-path etc. arguments.
• Manual runtime verification against real archives — same gap as refactor(unrealengine): extract UE plugin + wire DI into IPluginContext #25. PSKImporter and MDLExporter are the most-exercised real-world paths and I cannot validate them from CI. Before merge, please:
  • Open the UI, convert a UE4 asset to MDL, confirm the Source compile workspace generates
  • Convert an OPT or MOW asset to MDL too, confirm the existing non-plugin path still works

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