feat(explainer): FlowMaster Explainer scene from Hindsight + EA2 doctrine

Eight-card explainer covering Business-as-Code philosophy, the five
element types (Flow/Data/View/Rule/Version), the DEFINE/VERSION/DEPLOY/
EXECUTE lifecycle, hubs, work items, EA2 graph storage, and the Pi
agent. Sourced from FlowMaster Element Architecture v2.1 + the EA2
collection/edge contracts recalled from Hindsight today.

New 'What is FlowMaster?' chip on Landing routes here.

src/scenes/Landing.tsx

@@ -82,6 +82,7 @@ export default function Landing() {
             <button className="hub-chip" onClick={() => setScene("geo-attendance")}>Attendance Map</button>
             <button className="hub-chip" onClick={() => setScene("agent")}>Talk to Pi</button>
             <button className="hub-chip" onClick={() => setScene("chat")}>Team Chat</button>
+            <button className="hub-chip" onClick={() => setScene("explainer")}>What is FlowMaster?</button>
           </div>
 
           <div className="hero-stats">