feat(atlas): Phase B tail — live namespaces + trim inline fallback

NamespaceSummary (test-first) renders the substrate ns line from live cluster
namespaces (system-filtered, sorted, capped). Handler now overlays both node
specs and the ns line via a single cached liveOverlay(). Removed the inline
SUBSTRATE/STAGES/NS data — /api/atlas.json is the single source; the page shows
an error banner on fetch failure instead of stale data.

GPU model naming intentionally NOT done: koala's node has no GPU product label
(only nvidia.com/gpu count), so "1× GPU" is the API's truth — naming the RTX 5070
would need GPU-feature-discovery, out of scope.

Verified: build/vet/lint(0)/test green.

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

internal/atlas/cluster_test.go

@@ -2,6 +2,7 @@ package atlas_test
 
 import (
 	"reflect"
+	"strings"
 	"testing"
 
 	"git.d-ma.be/mathias/cad-atlas/internal/atlas"
@@ -62,6 +63,41 @@ func TestMergeSubstrate_NoLiveReturnsAuthored(t *testing.T) {
 	}
 }
 
+func TestNamespaceSummary_FiltersSystemSortsAndJoins(t *testing.T) {
+	ns := []byte(`{"items":[
+	  {"metadata":{"name":"gitea"}},
+	  {"metadata":{"name":"kube-system"}},
+	  {"metadata":{"name":"ai-stack"}},
+	  {"metadata":{"name":"default"}},
+	  {"metadata":{"name":"flux-system"}},
+	  {"metadata":{"name":"brain"}}
+	]}`)
+
+	got, err := atlas.NamespaceSummary(ns)
+	if err != nil {
+		t.Fatalf("NamespaceSummary: %v", err)
+	}
+	if want := "ns: ai-stack · brain · gitea"; got != want {
+		t.Fatalf("summary = %q, want %q", got, want)
+	}
+}
+
+func TestNamespaceSummary_CapsWithMore(t *testing.T) {
+	var items []string
+	for i := 0; i < 15; i++ {
+		items = append(items, `{"metadata":{"name":"app`+string(rune('a'+i))+`"}}`)
+	}
+	ns := []byte(`{"items":[` + strings.Join(items, ",") + `]}`)
+
+	got, err := atlas.NamespaceSummary(ns)
+	if err != nil {
+		t.Fatalf("NamespaceSummary: %v", err)
+	}
+	if !strings.HasSuffix(got, "· +3 more") {
+		t.Fatalf("expected cap suffix, got %q", got)
+	}
+}
+
 func TestHostsFromNodes_ErrorsOnBadJSON(t *testing.T) {
 	if _, err := atlas.HostsFromNodes([]byte("{not json")); err == nil {
 		t.Fatal("expected error on bad JSON, got nil")