refactor(unrealengine): extract UE plugin + wire DI into IPluginContext

[dhkatz]

Summary

Continuation of #24's plugin-foundation work. Unreal Engine 4/5 support now lives in a standalone GMConverter.UnrealEngine plugin assembly. The core CLI and UI ship without CUE4Parse or MessagePack — those move with the plugin.

Plugin extraction

• New GMConverter.UnrealEngine project referencing only GMConverter.SDK, CUE4Parse, MessagePack, and ImageSharp. 33 files moved from Core: PSKImporter, UE4Explorer, UE2Explorer, UE4ScanCache, UE4ExportCache, all of Formats/{Unreal,PSK,PSA}/, plus the UE-specific helpers ExportSessionCache, DecodedImage, BarycentricRasterizer.
• UnrealEnginePlugin : IPlugin entry registers PSK + UE4/UE2 explorers via DI. plugin.json manifest at the plugin root.
• Core GMConverter.csproj drops CUE4Parse ProjectReferences and the MessagePack PackageReference — both move to the plugin.

DI on IPluginContext (slim API surface)

• SDK adds Microsoft.Extensions.DependencyInjection.Abstractions.
• IPluginContext now exposes IServiceProvider Services plus generic Register<T>() overloads alongside the direct-instance overloads. The generics activate the type via ActivatorUtilities.CreateInstance(provider), resolving constructor parameters from the host's service container.
• Named TextureFactory/LoggerFactory context properties removed — single source of truth via Services. Adding a new host service is now a one-line AddSingleton in PluginLoader with zero impact on existing plugins.
• IPlugin stays an interface (not a base class) to preserve single-inheritance, testability, SDK-as-pure-contract, and consistency with IImporter/IExporter/IExplorer.
• PSKImporter, UE4Explorer, UE2Explorer marked public so reflection-based activation works cleanly across the assembly boundary.

Shared utilities promoted to SDK

• PerfTimer (75+ call sites split across host and UE plugin) moved to GMConverter.SDK.Common.PerfTimer.
• ExplorerFileSystem (used by Core's MOW/Generic explorers and the UE plugin) moved to GMConverter.SDK.Explorer.ExplorerFileSystem.

Host bundling

• GMConverter.CLI.csproj and GMConverter.UI.csproj opt into the plugin via a <BundledPlugin> item with a PluginId metadata. A loose ProjectReference (ReferenceOutputAssembly=false) enforces build order; a CopyBundledPlugins AfterTargets="Build" target stages the plugin's bin/ into the host's bin/plugins/<id>/ folder.
• Plugin csproj knows nothing about host paths — after a plugin moves to its own repo, the host removes the BundledPlugin entry; end users get the plugin via a release artifact instead.

Native library handling

PluginLoadContext.LoadUnmanagedDll now falls back to a sibling-folder probe (with platform-conventional shared-library names) when AssemblyDependencyResolver returns null. CUE4Parse-Natives.dll is emitted by CUE4Parse.csproj as Content rather than the runtimes/<rid>/native/ NuGet layout, so deps.json does not list it as a native asset; the fallback closes that gap. Empirically verified: the deployed bin/.../plugins/unrealengine/ contains CUE4Parse-Natives.dll alongside the managed assemblies.

Host-side surface cleanup

• ConversionService (UI) and Program.cs (CLI) drop the hardcoded "psk" case from their input-format switches; resolution falls through to PluginHost.Registry.GetImporter() and surfaces a clearer error when the format is not recognized.
• ExplorerService drops UE4Explorer/UE2Explorer from its built-in list; both come through PluginHost.Registry.Explorers now.
• ConvertViewModel keeps a hardcoded "psk" display entry behind a TODO — dynamic projection over built-ins + plugin registry is out of scope for this PR.

Test plan

• dotnet build GMConverter.slnx --configuration Release → 0 warnings, 0 errors
• dotnet format GMConverter.slnx --verify-no-changes --severity warn → clean
• Empirical: bin/Release/net10.0/plugins/unrealengine/ populated under both CLI and UI with all expected DLLs including CUE4Parse-Natives.dll
• Manual runtime verification required: PSKImporter is ~1300 lines and is the most-exercised real-world code path. The refactor is mechanical (move + constructor injection + replace 2 new ImageSharpTexture(...) sites with ITextureFactory.FromRgba) but I could not run it against a real Fortnite/UE4 archive. Before merge, please:
  • Open the UI, browse a UE4 archive via the Explorer, confirm assets list correctly
  • Convert a sample asset to glTF, confirm output matches a pre-refactor baseline
  • Convert to MDL, confirm Source compile workspace generates

Notes

This unblocks the eventual extraction of GMConverter.UnrealEngine into its own repo under the new GMConverter org. The plugin's csproj is self-contained — no shared Plugin.props, no host-path coupling. The remaining migrations (Source as an output plugin, Frostbite from its parked branch) follow the same shape.

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