Reduce peak memory during edge grouping

Delete edges from the input map while iterating to free memory
as the grouped map is built. This halves peak memory usage during
groupEdgesByIdentityDigest (~1.4GB to ~700MB with ~1M edges).

Safe because Go allows map deletion during range, and all callers
do not use the edges map after grouping.

Signed-off-by: Sascha Grunert <sgrunert@redhat.com>

Author: saschagrunert

internal/promoter/image/sign.go

@@ -548,6 +548,8 @@ func groupEdgesByIdentityDigest(edges map[promotion.Edge]any) [][]promotion.Edge
 	grouped := make(map[key][]promotion.Edge, len(edges)/2)
 
 	for edge := range edges {
+		delete(edges, edge) // free as we go to reduce peak memory
+
 		// Skip metadata layers
 		if strings.HasSuffix(string(edge.DstImageTag.Tag), ".sig") ||
 			strings.HasSuffix(string(edge.DstImageTag.Tag), ".att") ||