ui/phase-13: planet inspector — read-only

Plumbs the map → inspector pathway: a click on a planet selects it
through the new SelectionStore, the sidebar Inspector tab swaps
its empty-state copy for a per-kind read-only field set, and a
mobile-only bottom-sheet mirrors the same content over the map.
Field projection in api/game-state.ts now surfaces every documented
planet field.

ui/frontend/tests/selection-store.test.ts

@@ -0,0 +1,47 @@
+// SelectionStore unit tests. The store is in-memory only and carries
+// no async lifecycle, so the cases focus on the rune transitions and
+// the post-`dispose` no-op contract.
+
+import { describe, expect, test } from "vitest";
+
+import { SelectionStore } from "../src/lib/selection.svelte";
+
+describe("SelectionStore", () => {
+	test("initial state has no selection", () => {
+		const store = new SelectionStore();
+		expect(store.selected).toBeNull();
+	});
+
+	test("selectPlanet records the planet id", () => {
+		const store = new SelectionStore();
+		store.selectPlanet(42);
+		expect(store.selected).toEqual({ kind: "planet", id: 42 });
+	});
+
+	test("selectPlanet replaces the previous selection", () => {
+		const store = new SelectionStore();
+		store.selectPlanet(1);
+		store.selectPlanet(2);
+		expect(store.selected).toEqual({ kind: "planet", id: 2 });
+	});
+
+	test("clear resets the selection to null", () => {
+		const store = new SelectionStore();
+		store.selectPlanet(7);
+		store.clear();
+		expect(store.selected).toBeNull();
+	});
+
+	test("dispose blocks subsequent mutations", () => {
+		const store = new SelectionStore();
+		store.selectPlanet(3);
+		store.dispose();
+		expect(store.selected).toBeNull();
+
+		store.selectPlanet(4);
+		expect(store.selected).toBeNull();
+
+		store.clear();
+		expect(store.selected).toBeNull();
+	});
+});