fix(kiwi): guard alias resolution against missing GUID

Fixes #192 — val.alias can exist as a stub without a guid in real
.fig files, causing guidToString(undefined) to crash on import.

packages/core/src/kiwi/fig-import.ts

@@ -52,7 +52,7 @@ function buildVariableColorResolver(
     const val = entry.variableData.value
     if (!val) return null
     if (val.colorValue) return val.colorValue
-    if (val.alias) return resolve(val.alias.guid, depth + 1)
+    if (val.alias?.guid) return resolve(val.alias.guid, depth + 1)
     return null
   }
 }