chore: replace CSharpier with dotnet format

.config/dotnet-tools.json

@@ -1,13 +0,0 @@
-{
-  "version": 1,
-  "isRoot": true,
-  "tools": {
-    "csharpier": {
-      "version": "1.3.0",
-      "commands": [
-        "csharpier"
-      ],
-      "rollForward": false
-    }
-  }
-}

.csharpierignore

@@ -1,4 +0,0 @@
-# CSharpier formats only C# sources; project/config XML keeps its layout.
-**/*.csproj
-**/*.config
-**/*.xml

.editorconfig

@@ -1,10 +1,10 @@
 # Unified code style for OpenNest.
-# Canonical formatter: CSharpier (see .config/dotnet-tools.json).
-#   dotnet tool restore
-#   dotnet csharpier check .    # verify before committing
-#   dotnet csharpier format .   # fix
-# These settings match CSharpier's conventions so IDE auto-formatting
-# (VS / Rider / VS Code) stays consistent with the formatter.
+# Canonical formatter: dotnet format.
+# Format only the files or directories being changed:
+#   dotnet format OpenNest.sln --include path/to/changed-file.cs
+#   dotnet format OpenNest.sln --include path/to/changed-file.cs --verify-no-changes
+# These settings are used by dotnet format and IDE auto-formatting
+# (VS / Rider / VS Code).
 
 root = true
 
@@ -25,8 +25,8 @@ indent_size = 2
 [*.{cs,csx}]
 indent_style = space
 indent_size = 4
-# CSharpier wraps code at 100 columns; dotnet format cannot re-wrap, but
-# IDEs surface a visual guide and analyzers can flag hard violations.
+# dotnet format cannot re-wrap source to this limit, but IDEs can surface it
+# as a visual guide and analyzers can flag hard violations.
 max_line_length = 100
 
 # --- Using directives (System first, outside the namespace) ---

README.md

@@ -67,11 +67,11 @@ dotnet build OpenNest.sln
 
 ### Code formatting
 
-C# sources are formatted with [CSharpier](https://csharpier.com/), pinned in `.config/dotnet-tools.json`; the matching style (4-space indent, Allman braces, System-first usings, 100-column wraps) is mirrored in `.editorconfig` so IDE auto-format agrees. Before committing:
+C# sources are formatted with [`dotnet format`](https://learn.microsoft.com/dotnet/core/tools/dotnet-format), using the repository's `.editorconfig` settings so IDE auto-formatting stays consistent. Format only the files or directories you change, rather than reformatting the full solution:
 
 ```bash
-dotnet tool restore
-dotnet csharpier format .    # apply; use `check` instead of `format` to verify only
+dotnet format OpenNest.sln --include path/to/changed-file.cs                         # apply
+dotnet format OpenNest.sln --include path/to/changed-file.cs --verify-no-changes     # verify only
 ```
 
 ### Cross-platform engine contract tests