Extract inline @code blocks from vt headers (size_report.h, modes.h,
sgr.h, paste.h, mouse.h, key.h) into standalone buildable examples
under example/. Each header now uses Doxygen @snippet tags to include
code from the example source files, keeping documentation in sync
with code that is verified to compile and run.
New example projects: c-vt-size-report and c-vt-modes. Existing
examples (c-vt-sgr, c-vt-paste, c-vt-mouse-encode, c-vt-key-encode)
gain snippet markers so their code can be referenced from the headers.
Conceptual snippets in key.h, mouse.h, and key/encoder.h that show
terminal-state usage patterns remain inline since they cannot be
compiled standalone.
Add a new c-vt-mouse-encode example that demonstrates how to use the
mouse encoder C API. The example creates a mouse encoder configured
with SGR format and normal tracking mode, sets up terminal geometry
for pixel-to-cell coordinate mapping, and encodes a left button press
event into a terminal escape sequence.
Mirrors the structure of the existing c-vt-key-encode example. Also
adds the corresponding @example doxygen reference in vt.h.
