Add post-quantum wallet support and PQ transaction signing

app/src/main/cpp/pq/sphincsplus/randombytes.h

@@ -0,0 +1,15 @@
+#ifndef VEXTA_PQ_RANDOMBYTES_H
+#define VEXTA_PQ_RANDOMBYTES_H
+#include <stddef.h>
+#include <stdint.h>
+#ifdef __cplusplus
+extern "C" {
+#endif
+/* VEXTA: unified prototype. Definition lives in pq_randombytes.cpp and is
+   backed by Bitcoin's CSPRNG (GetStrongRandBytes). The upstream test-only
+   randombytes.c files were deleted — they must never ship in a wallet. */
+void randombytes(uint8_t *out, size_t outlen);
+#ifdef __cplusplus
+}
+#endif
+#endif