Initial scaffold: spec, architecture playbook, and M0 skeleton

src/app/App.tsx

@@ -0,0 +1,33 @@
+import styles from './App.module.css';
+
+/**
+ * Application shell — the three-pane workspace from spec §01A
+ * (library · editor · preview) under a fixed header.
+ *
+ * This is the skeleton: panes are placeholders. Each milestone fills one in
+ * (see docs/IMPLEMENTATION-PLAN.md). Resizing, toggling, modals, routing, and
+ * shortcuts arrive in later milestones.
+ */
+export function App() {
+  return (
+    <div className={styles.app}>
+      <header className={styles.header}>
+        <span className={styles.title}>Astrolabe</span>
+        <span className={styles.version}>v{__APP_VERSION__}</span>
+        <span className={styles.spacer} />
+      </header>
+
+      <main className={styles.panes}>
+        <section className={styles.pane} aria-label="Snippet library">
+          <div className={styles.paneLabel}>Library</div>
+        </section>
+        <section className={styles.pane} aria-label="Spec editor">
+          <div className={styles.paneLabel}>Editor</div>
+        </section>
+        <section className={styles.pane} aria-label="Live preview">
+          <div className={styles.paneLabel}>Preview</div>
+        </section>
+      </main>
+    </div>
+  );
+}