Type Resonite texture payload variants

[esnya]

Summary

• Split target-internal ResoniteTexturePayload into RawRgba32ResoniteTexturePayload and EncodedImageResoniteTexturePayload so raw payload dimensions and bytes are available without nullable fallback.
• Removed ResoniteTexturePayloadFormat and switched target/test adapters to concrete payload pattern matching.
• Removed the remaining null-forgiving encoded-source handoff in the neutral texture contract and target payload constructors.

Notes

• The target payload variants are classes rather than records so the hierarchy stays closed by private protected construction without record copy-constructor escape, and so existing reference-identity behavior stays unchanged.
• This is not a same-shape wrapper split: raw payloads carry non-null Width, Height, and BinaryPayload; encoded payloads carry an import source and optional metadata dimensions.

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 for plateau-13213-higashimurayama-shi-2020 / 53395325 / dem,bldg / grid completed. git diff --no-index against runtime/windows/resonite/semantic-baselines/type-dem-grid-projection-result/baseline-parent/higashi-53395325-dem-bldg-grid-geotiff.json showed the existing parent-stack dedicated-material delta (96 +75/-21), with no new texture-payload-shaped regression. The generated dump was deleted and is not committed.

Review Gate

• Base PR Type texture payload variants #353 is CLEAN, CI green, has PR-body THUMBS_UP, and has no review threads.
• Parent PR Type Resonite material asset bindings #352 is CLEAN and THUMBS_UP; GitHub still reports one old non-outdated unresolved thread for the dedicated-material issue that this stack already changes. This PR is based on Type texture payload variants #353 and does not add new material binding behavior.

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Superseded by #340: these texture source/payload changes depend on the same upstream texture boundary and have been folded into the main-target #340 PR instead of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