Create a container's parent directory on write

A store with no enrolments has no group directory, and open(O_CREAT) creates
the file but never its parent. A first enrolment into a fresh store therefore
failed with ENOENT, which QTEE reports as an I/O error -- indistinguishable
from a real storage fault, and the same -5 a refused RPMB write produces.

implementations/main.cpp

@@ -337,6 +337,15 @@ void ServeGpFile(std::span<std::byte> sb) {
             sfs::WriteReply(sb, EROFS, 0);
             return;
         }
+        // The group directory may not exist yet -- a store with no enrolments
+        // has no group at all, and open(O_CREAT) creates the file, never its
+        // parent. Without this a first enrolment into a fresh store fails with
+        // ENOENT, which QTEE reports as an I/O error indistinguishable from a
+        // real storage fault.
+        std::error_code ec;
+        std::filesystem::create_directories(
+            std::filesystem::path(*full).parent_path(), ec);
+
         // O_RDWR | O_CREAT | O_SYNC and never O_TRUNC: QTEE writes a container
         // as write(0,4096), write(4096,N), write(0,4096), so truncating on open
         // leaves 4096 bytes where a 258850-byte template belongs.