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/App.tsx

@@ -12,6 +12,7 @@ import Chat from "./scenes/Chat";
 import Agent from "./scenes/Agent";
 import Hub from "./scenes/Hub";
 import GeoAttendance from "./scenes/GeoAttendance";
+import Explainer from "./scenes/Explainer";
 import CommandBar from "./components/CommandBar";
 import Toaster from "./components/Toaster";
 import Console from "./components/Console";
@@ -165,6 +166,7 @@ export default function App() {
         {scene === "hub-hr" && <Hub hub="hr" />}
         {scene === "hub-it" && <Hub hub="it" />}
         {scene === "geo-attendance" && <GeoAttendance />}
+        {scene === "explainer" && <Explainer />}
         {scene === "settings" && <Settings />}
       </div>