Add point-programmable vector commitments.

A commitment can be published before its hidden coordinate is chosen.
Opening one side of each aligned 1-bit DPF pair sets that coordinate or
the vector sum, and a shift moves it onto a public index.

Co-authored-by: Cursor <cursoragent@cursor.com>

doc/examples.dox

@@ -102,4 +102,13 @@
 /// @example iterables/zip_iterable.cpp zip_iterable.cpp
 /// @brief an example of `dpf::zip_iterable` in use
 
-/// @}
+/// @}
+
+
+/// @{
+
+/// @example mwe/ppvc.cpp ppvc.cpp
+/// @brief a point-programmable vector commitment
+
+/// @}
+