Require raw texture dimensions at construction

[esnya]

Summary

• split in-memory texture source creation into raw and encoded entrypoints so raw textures require width/height at the call site
• make raw TexturePayload/ResoniteTexturePayload byte constructors dimension-required and encoded source-backed constructors format-fixed
• update mapper/test/canonical dump helpers to route raw vs encoded payloads through the typed entrypoints

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 (1006 passed)
• Fake Live Sink canonical dump for plateau-13213-higashimurayama-shi-2020, mesh 53395325, packages dem,bldg, terrain mesh grid matched the baseline with git diff --no-index; temp dump removed

[esnya]

レビュー対応として追加で TexturePayload / ResoniteTexturePayload の payload invariant を閉じました。

• raw/encoded の構築後に Width / Height / Format / Source / BinaryPayload を init で差し替えられないよう、payload の状態プロパティを get-only に変更
• それにより RawRgba32 なのに width/height が null という状態は通常の C# API から生成不能になったため、mapper/test support 側の Raw texture payload requires width/height runtime fallback を削除
• 新しい同型ラッパーは増やさず、既存の raw vs encoded constructor 境界をそのまま不変条件の境界にしています

検証:

• 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 (1006 passed)
• Fake Live Sink canonical dump: higashi-53395325-dem-bldg-grid-geotiff.json と一致

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Superseded by #340: these texture source/payload changes depend on the same upstream texture boundary and have been folded into the main-target #340 PR instead of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