fix(abi): do not error on the nothing-to-repair path (#208)

## Why

Pipeline 12480 shows this for slots with nothing to do:

```
Error in sprintf("%s/%s: %d objects recorded, none unsafe. Nothing to repair.",  :
```

`RPostgres` returns `count(*)` as `integer64`, and `%d` rejects it. So a slot in the **best possible state** — verdicts recorded, nothing unsafe — failed its pipeline step, while slots with real work to do succeeded. Exactly the wrong signal.

## What changed

Formats the count as a string. One line, plus a comment recording why `%d` is wrong here.

Reviewed-on: #208

local/list-unsafe-flat-objects.R

@@ -141,11 +141,13 @@ if (nrow(rows) == 0L) {
       call. = FALSE
     )
   }
+  # `count(*)` arrives as a double (or integer64), and `%d` rejects both, so the
+  # nothing-to-do path used to die on its own success message.
   message(sprintf(
-    "%s/%s: %d objects recorded, none unsafe. Nothing to repair.",
+    "%s/%s: %s objects recorded, none unsafe. Nothing to repair.",
     arch,
     codename,
-    total
+    format(as.numeric(total), scientific = FALSE)
   ))
   quit(status = 0)
 }