Add `.stack` property and `stack_string()` method to exceptions

[mjm918]

Summary

Add stack trace capture to exceptions for better debugging. Currently exceptions only have .message() method.

Sub-issues

This feature has been split into smaller, focused issues:

• Change exception .message() method to .message property #55 - Change .message() method to .message property (breaking change)
• Add stack_frame built-in type #56 - Add stack_frame built-in type
• Implement shadow stack runtime for stack traces #57 - Implement shadow stack runtime for stack traces
• Add .stack property to exceptions #58 - Add .stack property to exceptions
• Add stack_string() method to exceptions #59 - Add stack_string() method to exceptions

Dependency Graph

#55 (.message property)     #56 (stack_frame type)
         |                          |
         |                          v
         |                  #57 (shadow stack runtime)
         |                          |
         |                          v
         |                  #58 (.stack property)
         |                          |
         |                          v
         +------------------#59 (stack_string() method)

Implementation Order

1. Change exception .message() method to .message property #55 - .message() → .message (can be done independently)
2. Add stack_frame built-in type #56 - stack_frame type (foundation for stack traces)
3. Implement shadow stack runtime for stack traces #57 - Shadow stack runtime (depends on Add stack_frame built-in type #56)
4. Add .stack property to exceptions #58 - .stack property (depends on Add stack_frame built-in type #56, Implement shadow stack runtime for stack traces #57)
5. Add stack_string() method to exceptions #59 - stack_string() method (depends on Add .stack property to exceptions #58)

Final API

exception DivisionByZero {
    dividend: int,
}

fn main() {
    divide(10, 0) catch e {
        println(e.message);           // Property (changed from method)
        println(e.stack_string());    // Formatted stack trace
        
        // Or iterate frames:
        for (i: int, frame: stack_frame in e.stack) {
            println("{} at {}:{}", frame.function, frame.file, frame.line);
        }
    };
}

Completion Criteria

This tracking issue will be closed when all sub-issues are completed.

[mjm918]

Both .stack property and stack_string() method are implemented:

• e.stack returns [stack_frame] array
• e.stack_string() returns formatted stack trace string

See examples/exception_stack.naml for usage.