chore: add .editorconfig and pinned CSharpier tool manifest

Mirrors CSharpier conventions (4-space indent, Allman braces,
System-first usings, 100-col wraps) so IDE auto-format and
'dotnet format' agree with the canonical formatter.
Usage: dotnet tool restore && dotnet csharpier format .

.editorconfig

@@ -0,0 +1,68 @@
+# 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.
+
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[*.{csproj,props,targets,xml,config,manifest}]
+indent_style = space
+indent_size = 2
+
+[*.{json,yml,yaml}]
+indent_style = space
+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.
+max_line_length = 100
+
+# --- Using directives (System first, outside the namespace) ---
+dotnet_sort_system_directives_first = true
+csharp_using_directive_placement = outside_namespace:warning
+
+# --- Brace placement: Allman (opening brace on its own line) ---
+csharp_new_line_before_open_brace = all
+csharp_new_line_before_else = true
+csharp_new_line_before_catch = true
+csharp_new_line_before_finally = true
+csharp_new_line_before_members_in_object_initializers = true
+csharp_new_line_before_members_in_anonymous_types = true
+csharp_new_line_between_query_expression_clauses = true
+
+# --- Spacing ---
+csharp_space_after_keywords_in_control_flow_statements = true
+csharp_space_between_method_call_parameter_list_parentheses = false
+csharp_space_between_method_declaration_parameter_list_parentheses = false
+csharp_space_between_parentheses = false
+csharp_space_before_colon_in_inheritance_clause = true
+csharp_space_after_colon_in_inheritance_clause = true
+csharp_space_around_binary_operators = before_and_after
+csharp_space_after_cast = false
+csharp_space_after_comma = true
+csharp_space_before_comma = false
+
+# --- Code style preferences ---
+# Project rule: always use var for locals (see CLAUDE.md).
+csharp_style_var_for_built_in_types = true:suggestion
+csharp_style_var_when_type_is_apparent = true:suggestion
+csharp_style_var_elsewhere = true:suggestion
+csharp_prefer_braces = true:suggestion
+csharp_prefer_simple_using_statement = true:suggestion
+csharp_style_namespace_declarations = file_scoped:silent
+dotnet_style_prefer_auto_properties = true:suggestion
+dotnet_style_object_initializer = true:suggestion
+dotnet_style_collection_initializer = true:suggestion
+dotnet_style_prefer_is_null_check_over_reference_equality_method = true:suggestion