feat(userspace): add demo ring-3 programs

user/sbrktest.c

@@ -0,0 +1,27 @@
+#include "ulib.h"
+
+#define blockBytes 8192
+
+int main(void)
+{
+    print("sbrktest: pid=");
+    printDec(userGetpid());
+    print(" requesting 8192 bytes via sbrk\n");
+
+    char* block = (char*)userSbrk(blockBytes);
+
+    for (int i = 0; i < blockBytes; i++)
+        block[i] = (char)(i & 0x7F);
+
+    int intact = 1;
+    for (int i = 0; i < blockBytes; i++)
+        if (block[i] != (char)(i & 0x7F))
+            intact = 0;
+
+    if (intact)
+        print("sbrktest: heap page write and read-back verified OK\n");
+    else
+        print("sbrktest: heap verification FAILED\n");
+
+    return 0;
+}