fix(aiken-lang): include same-module function dependencies in UPLC export (#1333)

[knoal]

Problem

When exporting a function that calls a same-module private helper function, aiken export produces UPLC where the helper appears as a free variable reference rather than including its definition.

Repro from aiken-lang#1333:

pub fn helper(x: Int) -> Bool { x > 0 }
pub fn main(y: Int) -> Bool { helper(y) }

Running aiken export and evaluating the exported main with amaru-uplc gives:

OpenTermEvaluated(Var(DeBruijn(N)))

Root Cause

In hoist_function, when a same-module function dependency is hoisted onto the validator tree:

1. An Air::DefineFunc node is created with the function body embedded
2. The code generator produces the correct UPLC structure (lambda(func_name).apply(func_body) wrapping)
3. But: the function was never registered in key_to_func via insert_new_function

Without registration in key_to_func, the DefineFunc-generated UPLC creates a local variable binding that nothing resolves — leaving a free variable at the call site.

Fix

After hoisting a user-defined function (identified by being present in self.functions), register it in key_to_func via insert_new_function. This ensures the DefineFunc processing can look up the function and generate the correct Var reference.

The fix is guarded with self.functions.contains_key(key) to skip compiler-builtin functions (which use a different registration path).

Testing

• All 178 existing aiken-project --lib tests pass
• All 5 export tests pass
• Added regression test export_same_module_dependency in crates/aiken-project/src/export.rs

Fixes aiken-lang#1333