feat(diff): mark records deliberately kept outside the template

internal/diff/diff.go

@@ -19,6 +19,10 @@ type RecordDiff struct {
 	Desired  *model.Record // nil for Delete
 	Actual   *model.Record // nil for Add
 	ReadOnly bool          // NS/SOA — shown but never applied
+	// Custom marks a record the operator deliberately keeps outside the
+	// template: shown in its own section, never counted as drift, never
+	// applied. Set by MarkCustom, not by Diff.
+	Custom bool
 }
 
 // Key is the stable identifier of the RRset this diff targets, normalised the
@@ -37,7 +41,7 @@ type Changeset struct {
 func (c Changeset) Actionable() []RecordDiff {
 	var out []RecordDiff
 	for _, d := range c.Diffs {
-		if d.ReadOnly || d.Kind == InSync {
+		if d.ReadOnly || d.Custom || d.Kind == InSync {
 			continue
 		}
 		out = append(out, d)
@@ -52,7 +56,7 @@ func (c Changeset) Actionable() []RecordDiff {
 func (c Changeset) Updates() []RecordDiff {
 	var out []RecordDiff
 	for _, d := range c.Diffs {
-		if d.ReadOnly {
+		if d.ReadOnly || d.Custom {
 			continue
 		}
 		if d.Kind == Add || d.Kind == Update {
@@ -73,7 +77,7 @@ func (c Changeset) Updates() []RecordDiff {
 func (c Changeset) Prunes() []RecordDiff {
 	var out []RecordDiff
 	for _, d := range c.Diffs {
-		if d.ReadOnly {
+		if d.ReadOnly || d.Custom {
 			continue
 		}
 		if d.Kind == Delete {
@@ -131,3 +135,39 @@ func index(recs []model.Record) map[string]model.Record {
 	}
 	return m
 }
+
+// MarkCustom flags the diffs whose Key() is in keys as Custom. Only
+// Kind == Delete diffs are marked, and never read-only ones: the template
+// wins. As soon as the template starts describing a key, its diff becomes
+// Add/Update/InSync and the stored mark stops having any effect (it is not
+// deleted — the operator may go back to a template without that record).
+func (c *Changeset) MarkCustom(keys []string) {
+	if len(keys) == 0 {
+		return
+	}
+	set := make(map[string]bool, len(keys))
+	for _, k := range keys {
+		set[k] = true
+	}
+	for i := range c.Diffs {
+		d := &c.Diffs[i]
+		if d.ReadOnly || d.Kind != Delete {
+			continue
+		}
+		if set[d.Key()] {
+			d.Custom = true
+		}
+	}
+}
+
+// Customs returns diffs marked by MarkCustom. Disjoint from Updates() and
+// Prunes(), and outside Actionable() — a custom record is never drift.
+func (c Changeset) Customs() []RecordDiff {
+	var out []RecordDiff
+	for _, d := range c.Diffs {
+		if d.Custom {
+			out = append(out, d)
+		}
+	}
+	return out
+}