Counters: forget the totals of objects that no longer exist

hc counters kept listing the objects of a configuration replaced since
housecarld started (an import with --replace gives every object a new
ID) beside the current ones. Disabled objects, built-ins and policies
keep their totals.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/nft/nft.go

@@ -556,9 +556,39 @@ func (f *Firewall) Apply(ctx context.Context, st *model.State) error {
 			f.base[k] = b
 		}
 	}
+	pruneCounters(f.base, st)
 	return nil
 }
 
+// pruneCounters forgets the totals of objects st no longer has (deleted,
+// or replaced by an import with new IDs), so they aren't listed beside
+// the current ones. A disabled object keeps its totals for when it comes
+// back; built-ins and policies are always kept.
+func pruneCounters(base map[string]Counter, st *model.State) {
+	ids := map[string]bool{}
+	for _, r := range st.Rules {
+		ids[fmt.Sprintf("r%d", r.ID)] = true
+	}
+	for _, x := range st.Forwards {
+		ids[fmt.Sprintf("f%d", x.ID)] = true
+	}
+	for _, x := range st.SNATs {
+		ids[fmt.Sprintf("s%d", x.ID)] = true
+	}
+	for _, x := range st.Redirects {
+		ids[fmt.Sprintf("d%d", x.ID)] = true
+	}
+	for mark := range base {
+		obj, _, _ := strings.Cut(mark, "-") // f3-hairpin, f3-6 are f3's
+		if len(obj) < 2 || !strings.ContainsRune("rfsd", rune(obj[0])) || strings.Trim(obj[1:], "0123456789") != "" {
+			continue // b-established, p-input
+		}
+		if !ids[obj] {
+			delete(base, mark)
+		}
+	}
+}
+
 // SaveBoot writes the ruleset of st for the boot unit, and the fallback
 // it loads if that fails (FallbackFile), atomically. The boot ruleset has
 // no flowtable: it loads before the network comes up, when its devices