feat(debug): Add debug logging to console.log_handle for plugin investigation

- Added eprintln! debug messages to trace handle values
- Helps investigate why plugin return values display as blank
- Part of ongoing LLVM backend plugin return value investigation

Related to issue where print(c.get()) shows blank output

🤖 Generated with Claude Code

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

local_tests/test_plugin_debug.nyash

@@ -1 +1,11 @@
-print("Hello from Nyash\!")
+local c = new CounterBox()
+c.inc()
+local val = c.get()
+print("Before print:")
+print(val)
+print("After print")
+
+// 比較用
+local x = 42
+print("Normal int:")
+print(x)