Add a comprehensive "Effects" section to the terminal module
documentation in terminal.h explaining the callback system that
lets embedding applications react to terminal-initiated events
(bell, title changes, pty writes, device queries, etc.). The
section includes a reference table of all available effects and
their triggers, plus @snippet references to the new example.
Add c-vt-effects example project demonstrating how to register
write_pty, bell, and title_changed callbacks, attach userdata,
and feed VT data that triggers each effect.
