C#: Add documentation comment LST infrastructure

[jkschneider]

Summary

• Adds a structured LST for C# XML documentation comments (///) following the same architectural pattern as Java's Javadoc support
• CsDocComment interface with 8 node types (vs Javadoc's ~31) using a generic XML model since C# doc comments are XML-native
• CsDocCommentVisitor with bidirectional CSharpVisitor bridge for cref type references
• CsDocCommentPrinter and CsDocCommentParser for round-trip fidelity
• C# side groups consecutive /// lines into a single XmlDocComment in FormatWithComments
• RPC type mappings between C# XmlDocComment and Java CsDocCommentRawComment

Why not reuse Javadoc?

Javadoc models @param name desc positional syntax with ~31 tag-specific types. C# doc comments are XML (<param name="x">desc</param>) — fundamentally different structure. The architectural pattern is identical (separate tree hierarchy, DocComment implementing Comment, visitor with language visitor bridge via visitSpace()), but the node types are XML-centric.

Architecture

CsDocComment (interface extends Tree)
├── DocComment        — root; implements Comment for Space.comments attachment
├── XmlElement        — <tag>content</tag>
├── XmlEmptyElement   — <tag/>
├── XmlText           — plain text
├── XmlAttribute      — generic attribute
├── XmlCrefAttribute  — cref="Type" with J reference for type attribution
├── XmlNameAttribute  — name="param" with identifier
└── LineBreak         — line break with "/// " margin

Test plan

• Round-trip tests: parse C# with doc comments, print, verify identical output
• Visitor traversal tests for all node types
• Verify cref references get type attribution
• Edge cases: empty doc comments, nested XML, self-closing tags, multi-line content
• Run existing C# test suite for no regressions