Modernize switch statements to switch expressions

[craigsmitham]

Summary

Convert traditional switch statements to switch expressions (C# 8+) where each case returns or assigns a value, particularly in Printer.cs and similar AST-processing code.

Motivation

• Switch expressions are more concise and enforce exhaustiveness
• The Printer.cs switch on AST node types is a strong candidate — each case produces a value rather than performing complex side effects
• Aligns with modern idiomatic C# patterns already used elsewhere in the codebase

Tasks

• Audit switch statements in Printer.cs and other AST/visitor code
• Convert eligible switch statements to switch expressions
• Ensure exhaustiveness checks catch missing cases at compile time