[Subsystem] Implement Server-Side PTY Shell Process Manager (Port of Ivy.Hooks.Pty Backend)

Problem Statement

Rusty-Framework includes a frontend terminal.rs widget, but lacks the backend server-side PTY process lifecycle manager (port of Ivy.Hooks.Pty).

Without a server PTY process manager, the frontend terminal widget cannot connect to native OS shell sessions (bash/zsh/cmd/powershell) on the server.

Missing Functionality

• Server-side PTY process lifecycle manager (spawn, resize, write, signal, terminate).
• WebSocket binary/text streaming handler connecting PTY stdout/stdin/stderr to terminal.rs.
• Terminal window size synchronization (cols, rows, pixel dimensions).
• Process isolation and security execution sandboxing settings.

Proposed Architecture & Design

1. Integrate portable-pty or tokio-pty into rusty-server.
2. Add PTY session management handlers to the WebSocket routing module.
3. Handle terminal resize events (SIGWINCH / TIOCSWINSZ) received over WebSocket messages.
4. Provide security configuration options (allowed binaries, working directory restrictions, environment variable filtering).

Acceptance Criteria

• Server spawns native shell processes on Linux, macOS, and Windows.
• Interactive stdin/stdout data streams between PTY process and terminal.rs widget without latency or framing issues.
• Terminal resize messages dynamically adjust PTY dimensions.
• Clean process termination on WebSocket disconnect.

Start by reading the existing frontend terminal.rs widget and locating the rusty-server WebSocket routing module. Compare portable-pty and tokio-pty for spawning shells, streaming input and output, resizing, and termination across Linux, macOS, and Windows. Done means the acceptance criteria are met, including security configuration and cleanup on disconnect.