feat(atlas): UX sprint — progressive disclosure (Plain default + transitions)

Makes the atlas self-explanatory. Every stage/node gains a plain-language layer
(plain_title + plain "what happens" + jargon-free node text); the previous
technical copy demotes to a subtitle + on-demand detail. A Plain⇄Technical
toggle (default Plain, persisted) flips the whole atlas. Biggest win: every
transition arrow is now LABELLED with "what must be true to advance" (the gated-
flow story that was invisible), gate hops (human @04, CI @06) styled distinctly.
Spine repositioned into a uniform header band so labels never collide with copy.

Copy grounded in a fresh-eyes UX review (docs/UX-REVIEW.md, reviewer≠implementer).
Data model: plain_title/plain/trans_label/trans on Stage, plain on Node — guarded
by a test (every stage has plain_title + a transition). Live overlays unchanged.

Verified: build/vet/lint(0)/test green; Plain render screenshot-checked.

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

internal/atlas/build_test.go

@@ -41,6 +41,29 @@ func TestBuild_OverlaysCIStageNodesFromWorkflow(t *testing.T) {
 	}
 }
 
+func TestDefault_HasPlainLayerAndTransitions(t *testing.T) {
+	a, err := atlas.Build(atlas.DataJSON, []byte("jobs:\n  guard:\n    a: 1\n"))
+	if err != nil {
+		t.Fatalf("Build embedded atlas: %v", err)
+	}
+	for _, s := range a.Stages {
+		if s.PlainTitle == "" {
+			t.Fatalf("stage %s missing plain_title", s.No)
+		}
+		if s.TransLabel == "" {
+			t.Fatalf("stage %s missing trans_label", s.No)
+		}
+		if s.Generate == "ci-jobs" {
+			continue // nodes are generated live, no authored plain
+		}
+		for _, n := range s.Nodes {
+			if n.Plain == "" {
+				t.Fatalf("stage %s node %q missing plain", s.No, n.Title)
+			}
+		}
+	}
+}
+
 func TestBuild_ErrorsOnBadAtlasJSON(t *testing.T) {
 	if _, err := atlas.Build([]byte("{not json"), []byte("jobs:\n  x:\n    a: 1\n")); err == nil {
 		t.Fatal("expected error on bad atlas JSON, got nil")