#13182
Replace the OSC 52-specific kind and encoded payload callback with an
atomic clipboard write containing a normalized destination and decoded
MIME representations. This keeps protocol details out of embedders and
lets iTerm2 Copy use the same semantic path.
Represent clears with an empty content list, preserve binary payloads,
and return a generic result for protocols that acknowledge writes. Add
the C ABI descriptors, layout metadata, and effects example so future
multipart protocols can reuse the callback without another API break.
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.
