WriteAutoGeneratedHeader with a multi-line copyright writes the second and later lines as code, so the generated file doesn't compile #151

What's wrong

WriteAutoGeneratedHeader (CodeBlocker/CodeBlockerExtensions.cs:27-39) prefixes the whole copyright string with // once:

codeBlocker.WriteLine($"// {copyright}");

CodeBlocker.WriteLine(string) passes the text straight to IndentedTextWriter.WriteLine, so only the first line of a multi-line value is commented.

codeBlocker.WriteAutoGeneratedHeader("Copyright (c) 2026 Acme Corp.\nAll rights reserved.");

emits

// Copyright (c) 2026 Acme Corp.
All rights reserved.
// <auto-generated />

All rights reserved. is now top-level code, and the generated file fails to compile (CS0116 and similar). The same happens with \r\n, \r, NEL (U+0085), LINE SEPARATOR (U+2028) and PARAGRAPH SEPARATOR (U+2029), because the C# compiler treats all of them as line terminators.

Why it matters

• Copyright and licence notices are often more than one line, for example an SPDX identifier plus a holder line.
• Generators commonly forward the notice from configuration, such as .editorconfig's file_header_template, which is multi-line by convention.
• The header is the first thing written, so every generated file breaks.

#129 fixed this class of bug for doc comments: DocComment.SplitLines (CodeBlocker/Templates/DocComment.cs:233-234) splits on all five terminators. This helper was not covered by that fix.

RegionScope (CodeBlocker/Scopes.cs:114) has the same root cause: it writes #region {name} verbatim. A name containing a line break leaves the rest of the name on its own line as code.

Suggested fix

• Split copyright on the same terminators DocComment uses. Extract SplitLines into a shared internal helper.
• Write // {line} for each line, and a bare // for an empty line.
• For RegionScope, either reject a name containing a line terminator with ArgumentException, or write only up to the first terminator. Rejecting it is consistent with Unknown TypeKind or AccessorKind writes half a declaration before throwing, and an Expression accessor with no BodyFactory emits get => ; #142, which rejects bad input before writing anything.

Acceptance criteria

• WriteAutoGeneratedHeader("a\nb") emits // a, // b, // <auto-generated />. The same holds for each of \r\n, \r, U+0085, U+2028 and U+2029.
• A test covers each terminator.
• new RegionScope(cb, "a\nb") never produces output in which part of the name sits outside the #region directive.