add cycle detection with use

scripts/core/engine.js

@@ -45,8 +45,6 @@ prosperon.on('SIGSEGV', function() {
   os.exit(1)
 })
 
-var use_cache = {}
-
 Object.defineProperty(Function.prototype, "hashify", {
   value: function () {
     var hash = new Map()
@@ -67,6 +65,8 @@ var canonical = io.realdir(RESPATH) + 'resources.js'
 var content = io.slurp(RESPATH)
 var resources = js.eval(RESPATH, `(function setup_resources(){${content}})`).call({})
 
+var use_cache = {}
+
 use_cache[resources.canonical('resources.js')] = resources
 
 function print_api(obj) {
@@ -277,9 +277,49 @@ switch(os.platform()) {
     break
 }
 
+var use_cache = {}
+var inProgress = {}
+var loadingStack = []
+
 globalThis.use = function use(file) {
-  if (use_cache[file]) return use_cache[file]
+  // If we've already begun loading this file in this chain, show the cycle
+  if (loadingStack.includes(file)) {
+    // Find where in the stack this file first appeared
+    let cycleIndex = loadingStack.indexOf(file)
+    // Extract just the modules in the cycle
+    let cyclePath = loadingStack.slice(cycleIndex).concat(file)
+
+    throw new Error(
+      `Circular dependency detected while loading "${file}".\n` +
+      `Module chain: ${loadingStack.join(" -> ")}\n` +
+      `Cycle specifically: ${cyclePath.join(" -> ")}`
+    )
+  }
+
+  // Already fully loaded? Return it
+  if (use_cache[file]) {
+    return use_cache[file]
+  }
+
+  // If it's loading (but not on the stack), mark it as a new chain entry
+  // (This is optional if you just rely on loadingStack. 
+  //  But if you'd like a simple “already loading” check, keep 'inProgress'.)
+  if (inProgress[file]) {
+    throw new Error(`Circular dependency detected while loading "${file}"`)
+  }
+  inProgress[file] = true
+
+  // Push onto loading stack for chain tracking
+  loadingStack.push(file)
+
+  // Actually load the module
   var mod = script_fn(file)
+
+  // Done loading, remove from the chain and mark as loaded
+  loadingStack.pop()
+  delete inProgress[file]
+
+  // Cache and return
   use_cache[file] = mod.module_ret
   return use_cache[file]
 }