feat(workspace): add terminal panel with PTY and VT100 emulation

feat(workspace): add terminal panel with PTY and VT100 emulation

- New bridge.pty package: UnixPty (JNA-based), PtySession,
  Vt100Emulator, and ScreenBuffer supporting fullscreen curses apps
  (alternate screen, application cursor keys)
- New workspace package: Workspace and TerminalPanel render the
  terminal as a panel in the 3D workspace
- Main gains a --selftest mode that drives the PTY end-to-end
  (shell prompt, ls, arithmetic echo, mc/htop open+quit, ESC
  handling) for xvfb-run CI verification
- Build: Java 21, maven-compiler-plugin 3.13.0, sixth-3d
  1.5-SNAPSHOT, add JNA 5.14.0, drop javainspect, repo URL
  www2 -> www3
- Docs: reposition Sixth as a spatial computing environment, define
  roles/boundaries vs sixth-3d, outline VM/VNC and Xvfb roadmap