Establish reproducible formatting baseline

[Widthdom]

CLOSES #218

What

• Pin the formatting SDK to .NET SDK 8.0.423 with roll-forward disabled.
• Make .editorconfig the explicit formatting source of truth and define LF/CRLF plus generated-code handling.
• Add .gitattributes for deterministic checkout line endings and binary assets.
• Apply the one-time dotnet format baseline to all currently drifting C# files.
• Add a blocking formatting verification step to the main CI workflow.
• Document local verification and fix commands in English and Japanese.
• Add an architecture test that protects the SDK pin, formatting rules, line-ending policy, documentation, and CI gate.

Why

The repository previously used global.json with latestMinor, so formatting could resolve to different SDKs across developer machines and CI. The committed C# baseline also failed dotnet format --verify-no-changes, and neither CI nor contributor documentation prevented future drift.

Impact

This intentionally changes repository formatting and automation only. The mechanical C# baseline is whitespace-only and introduces no intended runtime behavior changes. CI will now reject formatting drift.

Validation

• dotnet format FolderDiffIL4DotNet.sln --verify-no-changes --no-restore --verbosity minimal
• dotnet build FolderDiffIL4DotNet.sln --configuration Release --no-restore --nologo -m:1
• dotnet test FolderDiffIL4DotNet.Tests/FolderDiffIL4DotNet.Tests.csproj --configuration Release --no-build --no-restore --nologo -m:1 — 2036 passed, 1 existing E2E skipped
• Focused formatting-baseline architecture test — passed
• git diff --check
• Adversarial review against origin/main — no remaining findings

Checklist

• I read AGENT_GUIDE.md.
• I used the nildiff command name in user-facing text.
• I ran the relevant tests.
• I ran the Release configuration test.
• I updated English and Japanese contributor documentation.
• I did not add secrets, tags, or publish steps.
• I used explicit git add <file> paths only.