feat(atlas): hybrid substrate — live specs for cluster nodes, authored rest

MergeSubstrate overlays live node specs onto the authored machine list by name:
cluster nodes (koala) get fresh live specs, non-cluster machines (iguana/flamingo/
piguard) stay authored, new live nodes are appended. Test-first. Restores the full
homelab machine list while keeping cluster nodes truthful.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/atlas/cluster.go

@@ -52,6 +52,33 @@ func HostsFromNodes(nodesJSON []byte) ([]Host, error) {
 	return hosts, nil
 }
 
+// MergeSubstrate overlays live node specs onto the authored substrate: an
+// authored host is replaced by the live node of the same name (fresh specs),
+// authored-only machines (non-cluster: iguana/flamingo/piguard) are kept, and
+// live-only nodes (new to the cluster) are appended.
+func MergeSubstrate(authored, live []Host) []Host {
+	liveByName := make(map[string]Host, len(live))
+	for _, h := range live {
+		liveByName[h.Name] = h
+	}
+	seen := make(map[string]bool, len(authored))
+	out := make([]Host, 0, len(authored)+len(live))
+	for _, a := range authored {
+		if l, ok := liveByName[a.Name]; ok {
+			out = append(out, l)
+		} else {
+			out = append(out, a)
+		}
+		seen[a.Name] = true
+	}
+	for _, l := range live {
+		if !seen[l.Name] {
+			out = append(out, l)
+		}
+	}
+	return out
+}
+
 // memGi converts a Kubernetes memory quantity in Ki (e.g. "67108864Ki") to a
 // rounded Gi string. Returns "" if unparseable.
 func memGi(ki string) string {