Undo reported "nothing left to undo" after a successful undo (#72)

Co-authored-by: Steve Dudenhoeffer <[email protected]>

internal/service/revisions.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"log/slog"
+	"sort"
 	"sync"
 
 	"gitea.stevedudenhoeffer.com/steve/pansy/internal/domain"
@@ -334,9 +335,48 @@ func (s *Service) RevertChangeSet(ctx context.Context, actorID, changeSetID int6
 	if err != nil {
 		return nil, nil, err
 	}
+	// Fill in the per-op counts. commitScope returns the freshly inserted row,
+	// which carries no tally — and a caller receiving a change set with an empty
+	// Counts cannot tell "nothing was reverted" from "the tally wasn't loaded".
+	// That ambiguity is not theoretical: it made the chat panel report a
+	// successful undo as "nothing left to undo".
+	if cs != nil {
+		cs.Counts = countRevisions(sc.taken())
+	}
 	return cs, conflicts, nil
 }
 
+// countRevisions tallies revisions by (entity type, op).
+//
+// This deliberately reproduces in Go what ListChangeSets does in SQL, because a
+// change set that has just been written has no rows to GROUP BY yet — the
+// alternative is a second round trip to count what we are already holding.
+// The parity is a real cross-layer contract, and TestRevertResultCarriesItsCounts
+// compares the two breakdowns row for row so it can't drift silently.
+func countRevisions(revs []domain.Revision) []domain.ChangeCount {
+	type key struct{ entityType, op string }
+	seen := map[key]int{}
+	order := []key{}
+	for _, r := range revs {
+		k := key{r.EntityType, r.Op}
+		if _, ok := seen[k]; !ok {
+			order = append(order, k)
+		}
+		seen[k]++
+	}
+	sort.Slice(order, func(i, j int) bool {
+		if order[i].entityType != order[j].entityType {
+			return order[i].entityType < order[j].entityType
+		}
+		return order[i].op < order[j].op
+	})
+	counts := make([]domain.ChangeCount, 0, len(order))
+	for _, k := range order {
+		counts = append(counts, domain.ChangeCount{EntityType: k.entityType, Op: k.op, N: seen[k]})
+	}
+	return counts
+}
+
 // entityKey identifies a row across the entity types a revision can name.
 type entityKey struct {
 	entityType string