Convert TreeNode from final class to struct #150

Context

TreeNode (introduced in PR #147 at Sources/SyntaxParser/TreeNode.swift) is declared as final class but behaves as a value type:

• Built up during a single traversal, then read-only
• Encoded to JSON immediately after construction
• Equatable conformance compares fields, not identity

The only genuine need for a reference type in this pipeline is the mutable current pointer inside TokenVisitor, which is already a separate internal type.

Proposed Work

Convert TreeNode to a struct. Key considerations:

• The parent: Int? back-reference is by ID (not object identity), so no reference cycle concerns
• Codable and Equatable synthesization works the same for structs
• TokenVisitor builds nodes by mutating a current node — this will need to capture the struct by value and reassign, rather than mutating through a reference