chore: import upstream snapshot with attribution

apps/sim/blocks/custom/overlay.ts

@@ -0,0 +1,36 @@
+import type { BlockConfig } from '@/blocks/types'
+
+/**
+ * Resolver for dynamic (DB-driven) custom blocks that live outside the static
+ * `BLOCK_REGISTRY`. The four core accessors in `@/blocks/registry` fall back to
+ * the registered resolver so custom block types resolve everywhere without
+ * rewriting the many synchronous `getBlock` call sites.
+ *
+ * Two environment-specific resolvers register here:
+ *  - client: a Map hydrated from `useCustomBlocks` (see `client-overlay.ts`)
+ *  - server: an AsyncLocalStorage map scoped per request/org (see `server-overlay.ts`)
+ *
+ * This module is isomorphic (no `'use client'`, no `node:` imports) so
+ * `registry.ts` stays importable on both sides.
+ */
+export interface BlockOverlayResolver {
+  get(type: string): BlockConfig | undefined
+  all(): BlockConfig[]
+}
+
+let resolver: BlockOverlayResolver | null = null
+
+/** Register (or clear with `null`) the active overlay resolver for this environment. */
+export function registerBlockOverlayResolver(next: BlockOverlayResolver | null): void {
+  resolver = next
+}
+
+/** Resolve a single custom block config by type, or `undefined` when none applies. */
+export function resolveOverlayBlock(type: string): BlockConfig | undefined {
+  return resolver?.get(type)
+}
+
+/** All custom block configs currently in scope (empty when no resolver is active). */
+export function overlayBlocks(): BlockConfig[] {
+  return resolver?.all() ?? []
+}