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/web/handler.go

@@ -38,7 +38,7 @@ func NewHandler() http.Handler {
 			return
 		}
 		if live := liveSubstrate(); len(live) > 0 {
-			a.Substrate = live
+			a.Substrate = atlas.MergeSubstrate(a.Substrate, live)
 		}
 		w.Header().Set("Content-Type", "application/json; charset=utf-8")
 		_ = json.NewEncoder(w).Encode(a)