Consolidate src/.editorconfig into root

Remove the redundant src/.editorconfig. Its two meaningful rules are
migrated to the root config:
- VSTHRD200 severity = none (don't require Async suffix on async methods)
- Verify snapshot files (*.received/*.verified) excluded from reformatting

The duplicate indent_size/charset rules are dropped (already set globally).
Fixed the stray quotes on the Verify charset value (utf-8-bom).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: hahn-kev

.editorconfig

@@ -15,6 +15,16 @@ indent_size = 2
 [*.{md,markdown}]
 trim_trailing_whitespace = false
 
+# Verify snapshot files — must not be reformatted or snapshot tests break.
+[*.{received,verified}.{txt,xml,json}]
+charset = utf-8-bom
+end_of_line = lf
+indent_size = unset
+indent_style = unset
+insert_final_newline = false
+tab_width = unset
+trim_trailing_whitespace = false
+
 # Vendored third-party code (C# port of Yjs) — excluded from formatting/analysis.
 # dotnet format skips files marked as generated.
 [src/Ycs/**.cs]
@@ -79,3 +89,7 @@ csharp_space_after_cast = false
 csharp_space_after_keywords_in_control_flow_statements = true
 csharp_space_between_method_declaration_parameter_list_parentheses = false
 csharp_space_between_method_call_parameter_list_parentheses = false
+
+# Analyzer diagnostics
+# VSTHRD200: do not require the "Async" suffix on async methods
+dotnet_diagnostic.VSTHRD200.severity = none