Type raw texture payload variants

[esnya]

Summary

• replace RawTexturePayloadFormat with concrete raw texture payload variants for RGBA32 and RGBA float32 payloads
• split raw texture source materialization into RGBA32 and RGBA float32 capabilities at source creation boundaries
• route transport, image loading, canonical dump, and tests through typed payload variants instead of runtime format checks

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
• Fake Live Sink canonical dump for plateau-13213-higashimurayama-shi-2020 / 53395325 / dem,bldg / grid; diff against runtime/windows/resonite/semantic-baselines/type-dem-grid-projection-result/baseline-parent/higashi-53395325-dem-bldg-grid-geotiff.json stayed at the known parent-stack 96 +75/-21 delta; temporary dump removed.

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Superseded by #340: these texture source/payload changes depend on the same upstream texture boundary and have been folded into the main-target #340 PR instead of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