enhancement: support mutual recursion (forward function references)

[t0k0sh1]

Summary

Functions cannot reference other functions defined later in the same file, making mutual recursion impossible.

Reproduction

function is_even(n: int) -> bool:
    if n == 0:
        return true
    return is_odd(n - 1)   # Error: undefined function: is_odd

function is_odd(n: int) -> bool:
    if n == 0:
        return false
    return is_even(n - 1)

Expected behavior

Functions should be able to call each other regardless of definition order, either via:

1. Two-pass resolution — collect all function signatures first, then resolve bodies
2. Forward declarations — explicit declare function is_odd(n: int) -> bool

Impact

Mutual recursion is a common pattern for state machines, parsers, and tree traversal algorithms.