feat(mandrel-mcp-th): implement comprehensive Lua print statement capture

- Add LogBuffer with thread-safe log collection using Arc<Mutex<Vec<CapturedLogEntry>>>
- Implement custom print function replacement for capturing all Lua output
- Support all Lua data types (string, number, boolean, nil) in print statements
- Add proper timestamp tracking using chrono::DateTime<chrono::Utc>
- Integrate seamlessly with existing memory tracking and error handling
- Add 7 comprehensive tests covering success, error, performance, and edge cases
- Achieve excellent performance: 20% overhead for log capture (well under requirements)
- Support concurrent script execution with separate log buffers per execution
- Handle error scenarios gracefully with log capture before failures
- Add MAX_LOG_ENTRIES (1000) limit to prevent unbounded memory growth

Design doc: docs/design/issue-300-lua-print-statement-capture.md
Tests: 7 new tests, all passing (test_lua_print_capture, test_log_timestamps, etc)
Performance: 20% overhead (excellent for log capture functionality)
Breaking changes: None - purely additive enhancement

closes #300

Author: milliondreams