Model XML documentation comments as structured data

[matt-edmondson]

Problem

In the template model being ported in #84, documentation is a bag of raw strings:

public List<string> Comments { get; set; } = [];

Callers therefore hand-assemble XML doc comments line by line, including the /// prefix and the tag markup. ktsu.Semantics even keeps named constants for the delimiters to avoid typos:

internal const string SummaryOpen = "/// <summary>";
internal const string SummaryClose = "/// </summary>";

Three consequences:

1. Nothing escapes the content. A description containing <, > or & — entirely plausible for a type described in metadata, e.g. "values in the range <0, 1>" — emits malformed XML and trips the compiler's doc-comment warnings.
2. Nothing enforces structure. A <param> tag whose name does not match any parameter produces CS1572/CS1573 in the generated file, and there is no way for the model to catch it even though it knows the parameter list.
3. Every generator re-derives the same layout — which tags in which order, how to wrap long lines, how to indent a multi-line <remarks>.

Proposal

Add a DocComment type carrying the standard tags, and render it from the template:

public sealed class DocComment
{
    public string? Summary { get; set; }
    public string? Remarks { get; set; }
    public string? Returns { get; set; }
    public string? Value { get; set; }
    public List<(string Name, string Text)> Params { get; }
    public List<(string Name, string Text)> TypeParams { get; }
    public List<(string Cref, string Text)> Exceptions { get; }
    public List<string> SeeAlso { get; }
    public string? InheritDoc { get; set; }   // renders /// <inheritdoc/>
}

with:

• XML-escaping of all text content by default, and an explicit opt-out for callers who are deliberately embedding markup such as <c> or <see cref="..."/>;
• canonical tag order (summary, typeparam, param, returns, value, exception, remarks, seealso);
• correct /// prefixing and indentation for multi-line text, honouring the configured indent string;
• optional validation against the owning template's parameter and type-parameter lists, surfaced as an exception or a diagnostic rather than as a compile error in the generated output.

Keep Comments as the escape hatch for non-doc comments and anything the model does not cover.

Acceptance criteria

• DocComment renders each supported tag in canonical order with correct prefixing and indentation.
• Text content is XML-escaped by default; the opt-out is tested.
• <param> / <typeparam> names can be validated against a MethodTemplate / ConstructorTemplate parameter list.
• Multi-line <remarks> and <summary> render each line with its own /// prefix.
• README.md documents the type with an example.

Context

Part of ktsu-dev/Semantics#181. Depends on #84.