Add C# symbol indexing support

[uchebnick]

Goal

Add first-class C# Tree-sitter indexing so unch can search common .NET codebases semantically.

Suggested coverage

• Namespaces, classes, interfaces, structs, records, enums
• Constructors, methods, properties, and events
• Generic type/method signatures where practical
• XML documentation comments and nearby comments

Acceptance criteria

• .cs files are parsed through Tree-sitter instead of fallback text prefixes.
• Tests cover representative namespace/class/interface/record/method/property examples.
• Search results include stable kind/name/qualified_name/signature fields.
• Parser failures fall back safely without breaking indexing.