Enhancement: Add const-correctness throughout codebase

[clank-hayen]

Problem

Several AST node fields are marked `mutable` to allow the typechecker to annotate them:

// In ast.hpp
struct FunctionCall : ASTNode {
    string name;
    vector<unique_ptr<ASTNode>> args;
    mutable string resolved_struct;  // Set by type checker
};

struct MethodCall : ASTNode {
    unique_ptr<ASTNode> object;
    string method_name;
    vector<unique_ptr<ASTNode>> args;
    mutable string object_type;  // Set by type checker
};

struct VariableDecl : ASTNode {
    mutable string type;  // May be updated by typechecker
    // ...
};

This breaks const-correctness and makes it unclear when/where these fields are set.

Current Pattern

1. Parser creates AST nodes with some fields empty
2. Typechecker fills in mutable fields during analysis
3. Codegen reads all fields

This creates implicit coupling between phases.

Suggested Solution

Option 1: Separate annotation data

struct TypeCheckAnnotations {
    map<const ASTNode*, string> object_types;
    map<const FunctionCall*, string> resolved_structs;
    map<const VariableDecl*, string> inferred_types;
};

string codegen::generate(const Program& prog, const TypeCheckAnnotations& annotations);

Option 2: Create annotated AST types

struct TypedMethodCall {
    const MethodCall& call;
    string object_type;
};

Option 3: Document the contract (minimal change)
Add clear documentation about which fields are parser-set vs typechecker-set.

Impact

• Cleaner phase separation
• Easier to reason about data flow
• Potential for parallel type checking