Fix User-Defined Functions (DEF FN)

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# Fix User-Defined Functions (DEF FN)

## Issue Description
The implementation of user-defined functions using the DEF FN syntax has issues. When trying to create and use a function, the interpreter either fails to recognize the syntax or throws an error during execution.

## Steps to Reproduce
Create a file with the following BASIC code:

Running this program results in either parse errors or runtime errors with messages like 'FNA is not an array'.

## Expected Behavior
- The DEF FN statement should define a user function
- The function should be callable using the syntax FN A(X)
- The function should execute correctly and return the expected result

## Technical Details
Issues may exist in multiple components:
1. Lexer/parser might not correctly handle the FN syntax
2. The interpreter may not be storing or retrieving the function definition correctly
3. Function evaluation might not be implemented properly

## Proposed Solution
1. Ensure the parser correctly handles both DEF FN and function call syntax
2. Update function storage mechanism in the environment
3. Fix function evaluation in the interpreter
4. Consider examining original ALTAIR BASIC documentation for the exact syntax

## Related Components
- src/core/lexer.cpp - Tokenization of FN syntax
- src/core/parser.cpp - Parsing function definitions and calls
- src/core/ast.cpp - Function representation
- src/core/interpreter.cpp - Function execution

## Priority
Medium - Functions are an important feature but not as critical as core execution flow

Start with src/core/lexer.cpp and src/core/parser.cpp to trace DEF FN and FN A(X) handling, then inspect src/core/ast.cpp and src/core/interpreter.cpp for function storage and evaluation. Reproduce the reported parse or “FNA is not an array” error with a minimal BASIC program. Done means DEF FN defines a function and FN A(X) calls it and returns the expected result.

Written by the indexing model from the issue text.