Per-Session GDB Sandbox Architecture with Real-Time Debugging for GDB-UI

GDB-UI currently supports only one user at a time because the backend shares a single global GDB controller across all requests. When two users debug simultaneously, the second session silently destroys the first with no error or warning. This proposal implements a three-phase solution. Phase 1 replaces the global controller with a thread-safe SessionManager that gives each user their own isolated GDB instance via a UUID session_id. A working implementation is already open as PR #131 with 15 passing tests, security fixes, and a live demo page. Phase 2 introduces WebSocket streaming with a per-session reader thread architecture so GDB output reaches the frontend in real time without blocking the server. Phase 3 moves each project into an isolated Docker container using the Docker SDK exec_start socket API for interactive GDB attachment, solving file descriptor exhaustion at the OS level. Deliverables: merged SessionManager with full test coverage, WebSocket streaming with reconnection handling, Docker sandbox with resource limits, security audit, and handover documentation.