thread context scratch system; converge on transition idiom in sample code

samples/static_site_generator/static_site_generator.c

@@ -34,6 +34,9 @@ static MD_Arena *arena = 0;
 
 int main(int argument_count, char **arguments)
 {
+    MD_ThreadContext tctx;
+    MD_ThreadInit(&tctx);
+    
     arena = MD_ArenaNew(1ull << 40);
     
     //~ NOTE(rjf): Parse command line arguments.