feat(atlas): UX — plain node titles + mobile transition rows (re-review #1,#2)

Lap-2 review's top two findings:
#1 Node titles were still jargon (the lap-1 disease one level down). Every node
now has a plain_t ("var-go Oath" → "Definition of done", "Admission controller"
→ "Tamper-proof seal", …); Plain view leads with it and demotes the technical
name to a dim in-card subtitle — mirrors the stage-head pattern. plain_t on every
authored node is now guard-tested.
#2 Transition labels lived only in the SVG spine (display:none <820px) with a
hover-only rationale — invisible on phones. Added stacked-layout transition rows
(HTML, always-visible, full sentence, no hover) shown on mobile; desktop header
band scoped to ≥821px so mobile isn't stretched.

build/vet/lint(0)/test green; desktop Plain screenshot-verified.

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

internal/atlas/build_test.go

@@ -60,6 +60,9 @@ func TestDefault_HasPlainLayerAndTransitions(t *testing.T) {
 			if n.Plain == "" {
 				t.Fatalf("stage %s node %q missing plain", s.No, n.Title)
 			}
+			if n.PlainTitle == "" {
+				t.Fatalf("stage %s node %q missing plain_t", s.No, n.Title)
+			}
 		}
 	}
 }