FFI functions returning Int64/UInt64 are not properly converted from JavaScript numbers

[BIYUEHU]

When using FFI to bind JavaScript functions that return numbers, the returned values are not properly converted to MoonBit's Int64/UInt64 types. Instead of performing the necessary conversion to the {hi, lo} object representation, the raw JavaScript number is passed through, causing incorrect behavior in MoonBit code. Reproducing Steps:

1. Define an FFI function that returns Int64 from a JavaScript function returning a number
2. Call the function and print the result using MoonBit's println
3. Compare with the same function returning Int type

Code

• Target: js

// FFI function returning Int64 - problematic
pub extern "js" fn p(s : String) -> Int64 = "parseInt"

// FFI function returning Int - works correctly  
pub extern "js" fn p1(s : String) -> Int = "parseInt"

fn main {
  // These should print the same values but don't
  println(p1("123"))        // Correctly prints: 123
  println(p("123"))         // Incorrectly prints: 0 (or garbage)
  
  // More examples showing the issue
  println(p1("122222222223"))   // Correctly prints: 122222222223 (if within Int range)
  println(p("122222222223"))    // Incorrectly prints: 0
}

Environment

• OS: Windows
• OS distribution and/or version: Windows11
• CPU Architecture: x86_64

moon 0.1.20250724 (2797252 2025-07-24) D:\Languages\moon\bin\moon.exe
moonc v0.6.22 D:\Languages\moon\bin\moonc.exe
moonrun 0.1.20250724 (2797252 2025-07-24) D:\Languages\moon\bin\moonrun.exe

Error output

The generated JavaScript shows the issue clearly:

// FFI functions - both call parseInt directly
const himeno$kana$45$rhythm$main$$p = (s)=>parseInt(s);   // Should return Int64 {hi, lo}
const himeno$kana$45$rhythm$main$$p1 = (s)=>parseInt(s);  // Returns Int (number)

// Int64 is represented as {hi, lo} object internally
const $0L = { hi: 0, lo: 0 };

// The problem: parseInt returns a JavaScript number, but MoonBit expects {hi, lo}
// No automatic conversion happens, leading to type mismatch

Console output shows incorrect values for Int64 functions while Int functions work correctly. Expected Behavior: FFI functions declared to return Int64/UInt64 should automatically convert JavaScript numbers to the appropriate {hi, lo} object representation, similar to how moonbitlang$core$builtin$$MyInt64$from_int works internally.

[peter-jerry-ye]

The Int64/UInt64 are defined structure instead of BigInt inside MoonBit due to performance issue.

[peter-jerry-ye]

We are not supporting it at FFI boundary at the moment, though it's a nice to have feature @hackwaly