Add roundtrip tests for COLOR variable alpha handling

Co-authored-by: Shai Rubinstein <rubinsh@gmail.com>

tests/engine/fig-export-variables.test.ts

@@ -0,0 +1,47 @@
+import { describe, test, expect, beforeAll } from 'bun:test'
+
+import {
+  exportFigFile,
+  parseFigFile,
+  initCodec,
+  SceneGraph,
+} from '@open-pencil/core'
+
+beforeAll(async () => {
+  await initCodec()
+})
+
+describe('COLOR variable alpha handling', () => {
+  test('COLOR variable without alpha exports successfully', async () => {
+    const graph = new SceneGraph()
+    const col = graph.createCollection('Colors')
+    // Simulate a COLOR variable value missing the alpha field
+    graph.createVariable('brand', 'COLOR', col.id, { r: 0.2, g: 0.4, b: 0.8 } as any)
+
+    const exported = await exportFigFile(graph)
+    const reimported = await parseFigFile(exported.buffer as ArrayBuffer)
+
+    const vars = [...reimported.variables.values()]
+    const colorVar = vars.find((v) => v.name === 'brand')!
+    expect(colorVar.type).toBe('COLOR')
+    const val = Object.values(colorVar.valuesByMode)[0] as { r: number; g: number; b: number; a: number }
+    expect(val.r).toBeCloseTo(0.2, 1)
+    expect(val.g).toBeCloseTo(0.4, 1)
+    expect(val.b).toBeCloseTo(0.8, 1)
+    expect(val.a).toBe(1)
+  })
+
+  test('COLOR variable with explicit alpha preserves it', async () => {
+    const graph = new SceneGraph()
+    const col = graph.createCollection('Colors')
+    graph.createVariable('overlay', 'COLOR', col.id, { r: 0, g: 0, b: 0, a: 0.5 })
+
+    const exported = await exportFigFile(graph)
+    const reimported = await parseFigFile(exported.buffer as ArrayBuffer)
+
+    const vars = [...reimported.variables.values()]
+    const colorVar = vars.find((v) => v.name === 'overlay')!
+    const val = Object.values(colorVar.valuesByMode)[0] as { r: number; g: number; b: number; a: number }
+    expect(val.a).toBeCloseTo(0.5, 1)
+  })
+})