feat: add shared rarity config and helpers

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

shared/lib/rarity.test.ts

@@ -0,0 +1,31 @@
+import { describe, it, expect } from "bun:test";
+import { getRarityConfig, RARITY_CONFIG, defaultName } from "./rarity";
+
+describe("getRarityConfig", () => {
+  it("returns correct config for known rarities", () => {
+    expect(getRarityConfig("SSR").color).toBe(0xF1C40F);
+    expect(getRarityConfig("SSR").emoji).toBe("🌟");
+    expect(getRarityConfig("SSR").label).toBe("SSR");
+  });
+
+  it("returns correct config for loot types", () => {
+    expect(getRarityConfig("CURRENCY").color).toBe(0x2ECC71);
+    expect(getRarityConfig("XP").color).toBe(0x1ABC9C);
+    expect(getRarityConfig("NOTHING").color).toBe(0x636363);
+  });
+
+  it("falls back to Common for unknown rarity", () => {
+    const result = getRarityConfig("LEGENDARY");
+    expect(result).toEqual(RARITY_CONFIG["C"]);
+  });
+});
+
+describe("defaultName", () => {
+  it("extracts filename from path", () => {
+    expect(defaultName("/assets/items/sword.png")).toBe("sword.png");
+  });
+
+  it("returns image.png for empty path", () => {
+    expect(defaultName("")).toBe("image.png");
+  });
+});