🗂️ Phase 9.75D: Remove duplicate traits.rs file - complete trait consolidation

## Summary
- Removed unused duplicate file: src/boxes/traits.rs (88 lines)
- Kept main trait definition file: src/box_trait.rs (904 lines, 60+ files using it)
- Completed trait system consolidation

## Analysis Results
- ❌ src/boxes/traits.rs: 88 lines, 0 files using it, missing share_box() method
- ✅ src/box_trait.rs: 904 lines, 60+ files using it, complete trait implementation

## Changes Made
- Deleted: src/boxes/traits.rs (outdated, incomplete, unused)
- Preserved: src/box_trait.rs (current, complete, widely used)

## Verification
- ✅ `cargo check` passes successfully (no errors, only warnings)
- ✅ No build dependencies broken
- ✅ All Box trait implementations remain functional
- ✅ Unified trait system with single source of truth

## Result
- Single authoritative trait definition file
- Eliminated confusion from duplicate implementations
- Cleaner codebase architecture
- Ready for Phase 9.75D VM/WASM backend development

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

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

test_simple_array2.nyash

@@ -0,0 +1,10 @@
+// 更にシンプルなArrayBoxテスト
+local arr
+arr = new ArrayBox()
+print("初期length: " + arr.length().toString())
+
+arr.push("item1")
+print("push後length: " + arr.length().toString())
+
+arr.push("item2")  
+print("2つ目push後length: " + arr.length().toString())