refactor: enforce import directions and expose source factories

src/sources/sourceSlot.js

@@ -0,0 +1,52 @@
+/** Bind a source contract once for the application without replacing its layer instance. */
+export function createSourceSlot(
+  initial,
+  methods,
+  label = 'Source',
+  optional = {},
+) {
+  let active = initial;
+  let binding = null;
+  const source = Object.fromEntries(
+    [...methods, ...Object.keys(optional)].map((method) => [
+      method,
+      (...args) => {
+        if (typeof active?.[method] !== 'function' && optional[method])
+          return optional[method](...args);
+        if (typeof active?.[method] !== 'function')
+          throw new Error(`${label} is not configured`);
+        const owner = binding;
+        const provider = active;
+        const value = provider[method](...args);
+        if (value && typeof value.then === 'function')
+          return value.then((result) => {
+            if (binding !== owner || active !== provider)
+              throw new DOMException(`${label} was replaced`, 'AbortError');
+            return result;
+          });
+        return value;
+      },
+    ]),
+  );
+  for (const property of ['label', 'attribution']) {
+    Object.defineProperty(source, property, {
+      enumerable: true,
+      get: () => active?.[property],
+    });
+  }
+  return {
+    source,
+    configure(next) {
+      if (methods.some((method) => typeof next?.[method] !== 'function'))
+        throw new TypeError(`Invalid ${label.toLowerCase()}`);
+      const owner = Symbol(label);
+      active = next;
+      binding = owner;
+      return () => {
+        if (binding !== owner) return;
+        active = null;
+        binding = null;
+      };
+    },
+  };
+}