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>

examples/mwe/ppvc.cpp

@@ -0,0 +1,26 @@
+#include <cstdint>
+#include <iostream>
+
+#include "dpf.hpp"
+
+// Complete program. A point-programmable vector commitment publishes the
+// commitment before the hidden coordinate is chosen.
+//
+//   c++ -std=c++17 -march=native -I include -I thirdparty examples/mwe/ppvc.cpp
+int main()
+{
+    using scheme = dpf::ppvc<std::uint8_t, 8>;
+
+    const auto pp = scheme::setup();
+    const auto [com, st] = scheme::commit(pp);
+
+    const std::uint8_t xi = 40;
+    const std::uint64_t tau = 0x5a;
+    const auto op = scheme::open(st, 0, tau, xi);
+    const auto x = scheme::eval(op);
+    const auto rotated = scheme::eval_rotated(op);
+
+    const bool ok = scheme::accept(pp, com, op, x, xi) && rotated[xi] == tau;
+    std::cout << rotated[xi] << "\n";
+    return ok ? 0 : 1;
+}