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/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;