Type texture payload variants

[esnya]

Summary

• Split neutral TexturePayload into RawRgba32TexturePayload and EncodedImageTexturePayload.
• Remove the contract-level TexturePayloadFormat discriminator and raw Width ?? throw / Height ?? throw path from SceneImportContractMapper.
• Update upstream appearance loading and test helpers to choose the concrete texture payload variant at creation time.

Verification

• dotnet restore PlateauResoniteLink.sln --locked-mode --disable-build-servers
• dotnet format whitespace . --folder --verify-no-changes
• dotnet build PlateauResoniteLink.sln --configuration Release --no-restore --disable-build-servers -m:1 -p:UseSharedCompilation=false
• dotnet test PlateauResoniteLink.sln --configuration Release --no-restore --verbosity normal -m:1 --disable-build-servers -p:UseSharedCompilation=false (1021 passed)
• Fake Live Sink canonical dump completed for plateau-13213-higashimurayama-shi-2020 mesh 53395325, packages dem,bldg, terrain mesh grid.
• git diff --no-index against the semantic baseline was non-empty only in the already-stacked PresentationCommon dedicated-material output from the parent branch; no texture payload variant-specific dump regression was observed. Temporary dump was deleted.

Notes

• This is intentionally not a same-shape wrapper: raw payloads are the only contract type with non-null Width, Height, and BinaryPayload; encoded image payloads carry source-backed image data with optional dimensions.
• Target-internal ResoniteTexturePayloadFormat remains a separate follow-up candidate because it is downstream of the neutral import contract boundary.

[esnya]

Superseded by #340: these texture source/payload changes depend on the same upstream texture boundary and have been folded into the main-target #340 PR instead of keeping a dependent stack.