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/model.go

@@ -11,9 +11,11 @@ type Host struct {
 	Spec string `json:"k"`
 }
 
-// Node is a card within a stage.
+// Node is a card within a stage. Plain is the jargon-free default text; Desc is
+// the technical detail shown on demand.
 type Node struct {
 	Title string   `json:"t"`
+	Plain string   `json:"plain,omitempty"`
 	Desc  string   `json:"d,omitempty"`
 	Pill  string   `json:"pill,omitempty"`
 	Cls   string   `json:"cls,omitempty"`
@@ -22,15 +24,22 @@ type Node struct {
 	Gate  bool     `json:"gate,omitempty"`
 }
 
-// Stage is one column of the pipeline. When Generate is set, its Nodes are
-// derived from a source at Build time rather than taken from the authored data.
+// Stage is one column of the pipeline. PlainTitle/Plain are the plain-language
+// default layer; Title/Path/Nodes[].Desc are the technical layer. TransLabel/
+// Trans annotate the outgoing transition (the arrow to the next stage): what
+// moves work forward and what must be true to advance. When Generate is set,
+// Nodes are derived from a live source at Build time.
 type Stage struct {
-	No       string `json:"no"`
-	Title    string `json:"title"`
-	Path     string `json:"path,omitempty"`
-	Cls      string `json:"cls,omitempty"`
-	Generate string `json:"generate,omitempty"`
-	Nodes    []Node `json:"nodes"`
+	No         string `json:"no"`
+	Title      string `json:"title"`
+	PlainTitle string `json:"plain_title,omitempty"`
+	Plain      string `json:"plain,omitempty"`
+	Path       string `json:"path,omitempty"`
+	Cls        string `json:"cls,omitempty"`
+	Generate   string `json:"generate,omitempty"`
+	TransLabel string `json:"trans_label,omitempty"`
+	Trans      string `json:"trans,omitempty"`
+	Nodes      []Node `json:"nodes"`
 }
 
 // Atlas is the full data model the frontend renders.