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/model.go

@@ -14,14 +14,15 @@ type Host struct {
 // 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"`
-	Tags  []string `json:"tags,omitempty"`
-	Risk  bool     `json:"risk,omitempty"`
-	Gate  bool     `json:"gate,omitempty"`
+	Title      string   `json:"t"`
+	PlainTitle string   `json:"plain_t,omitempty"`
+	Plain      string   `json:"plain,omitempty"`
+	Desc       string   `json:"d,omitempty"`
+	Pill       string   `json:"pill,omitempty"`
+	Cls        string   `json:"cls,omitempty"`
+	Tags       []string `json:"tags,omitempty"`
+	Risk       bool     `json:"risk,omitempty"`
+	Gate       bool     `json:"gate,omitempty"`
 }
 
 // Stage is one column of the pipeline. PlainTitle/Plain are the plain-language