feat(phase-9.75g-0): Complete BID-FFI Day 3 - FutureBox integration

- Add FutureBox BID bridge implementation for async Box types
- Support FutureBox in box_to_bid_handle conversion
- Add comprehensive FutureBox BID round-trip test
- Update CURRENT_TASK.md to reflect Day 3 completion (100%)

Day 3 fully completed! All Box types (String/Integer/Future) now integrated with BID-FFI.
Everything is Box philosophy proven through unified handle management! 🎉

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/bid/bridge.rs

@@ -90,6 +90,12 @@ pub fn box_to_bid_handle(
             arc_box.clone()
         );
         Ok((BidType::Handle { type_id: 2, instance_id: handle.instance_id }, handle))
+    } else if let Some(_future_box) = arc_box.as_any().downcast_ref::<crate::boxes::future::NyashFutureBox>() {
+        let handle = registry.register_box(
+            crate::bid::types::BoxTypeId::FutureBox as u32,
+            arc_box.clone()
+        );
+        Ok((BidType::Handle { type_id: 7, instance_id: handle.instance_id }, handle))
     } else {
         Err(BidError::InvalidType)
     }
@@ -199,4 +205,38 @@ mod tests {
         let retrieved_value = extract_integer_value(&retrieved).unwrap();
         assert_eq!(retrieved_value, 42);
     }
+    
+    #[test]
+    fn test_future_box_bid_conversion() {
+        let mut registry = BoxRegistry::new();
+        
+        // Create FutureBox
+        let future_box = crate::boxes::future::NyashFutureBox::new();
+        let arc_box: Arc<dyn NyashBox> = Arc::new(future_box);
+        
+        // Convert to BID handle
+        let (bid_type, handle) = box_to_bid_handle(&arc_box, &mut registry).unwrap();
+        assert_eq!(handle.type_id, 7); // FutureBox type ID
+        match bid_type {
+            BidType::Handle { type_id, .. } => assert_eq!(type_id, 7),
+            _ => panic!("Expected Handle type"),
+        }
+        
+        // Round-trip test
+        let retrieved = bid_handle_to_box(handle, &registry).unwrap();
+        
+        // Verify it's still a FutureBox
+        assert!(retrieved.as_any().downcast_ref::<crate::boxes::future::NyashFutureBox>().is_some());
+        
+        // Test with result set
+        if let Some(future) = arc_box.as_any().downcast_ref::<crate::boxes::future::NyashFutureBox>() {
+            let string_result = crate::boxes::string_box::StringBox::new("Future Result");
+            future.set_result(Box::new(string_result));
+            
+            // Verify state
+            assert!(future.ready());
+            let result = future.get();
+            assert_eq!(result.to_string_box().value, "Future Result");
+        }
+    }
 }

src/boxes/future/mod.rs

@@ -3,6 +3,7 @@
 // 参考: 既存Boxの設計思想
 
 use crate::box_trait::{NyashBox, StringBox, BoolBox, BoxCore, BoxBase};
+use crate::bid::{BidBridge, BidHandle, BidType, BidError, BoxRegistry};
 use std::any::Any;
 use std::sync::RwLock;
 
@@ -146,6 +147,25 @@ impl std::fmt::Display for NyashFutureBox {
     }
 }
 
+impl BidBridge for NyashFutureBox {
+    fn to_bid_handle(&self, registry: &mut BoxRegistry) -> Result<BidHandle, BidError> {
+        use std::sync::Arc;
+        let arc_box: Arc<dyn NyashBox> = Arc::new(self.clone());
+        let handle = registry.register_box(
+            crate::bid::types::BoxTypeId::FutureBox as u32,
+            arc_box
+        );
+        Ok(handle)
+    }
+    
+    fn bid_type(&self) -> BidType {
+        BidType::Handle { 
+            type_id: crate::bid::types::BoxTypeId::FutureBox as u32,
+            instance_id: 0  // Will be filled by registry
+        }
+    }
+}
+
 // Export NyashFutureBox as FutureBox for consistency
 pub type FutureBox = NyashFutureBox;