fix: separate logger from dashboard output

Logger writes to stderr so it doesn't corrupt the dashboard on stdout.
Dashboard tracks its own line count and uses cursor-up instead of home
so log messages appear cleanly before/after instead of interleaved.

celrs/log_write.c

@@ -2,5 +2,5 @@
 #include <stdio.h>
 
 void cel_log_write(char const* msg) {
-    printf("%s\n", msg);
+    fprintf(stderr, "%s\n", msg);
 }