fix(selfhost): ship the platform workerd binary in the docker runtime (#1394)

The workerd npm package's bin/workerd is a Node shim that execs the real
binary from the per-platform optional dependency
(@cloudflare/workerd-<os>-<arch>), which package-runtime.ts never copied
into the image. Custom app tools failed to sync or invoke with 'workerd
is unavailable on this platform'. Copy the platform package alongside the
shim, derived per build platform like the libsql native addon.

Author: RhysSullivan

.changeset/selfhost-workerd-platform-binary.md

@@ -0,0 +1,5 @@
+---
+"executor": patch
+---
+
+Ship the platform workerd binary in the self-host Docker runtime; without it custom app tools failed to sync or invoke with "workerd is unavailable on this platform".

apps/host-selfhost/scripts/package-runtime.ts

@@ -36,6 +36,30 @@ const libsqlNativePackage = (): string => {
   return `@libsql/${target}`;
 };
 
+// The `workerd` package's bin/workerd is a Node shim that execs the real
+// binary from the per-platform optional dependency
+// (`@cloudflare/workerd-<os>-<arch>`); without it the runtime throws
+// "workerd is unavailable on this platform" on first app-tool bundle or
+// invoke. Same per-platform shape as libsql above.
+const workerdPlatformPackage = (): string => {
+  const platformMap: Record<string, string> = {
+    "darwin-arm64": "workerd-darwin-arm64",
+    "darwin-x64": "workerd-darwin-64",
+    "linux-arm64": "workerd-linux-arm64",
+    "linux-x64": "workerd-linux-64",
+    "win32-x64": "workerd-windows-64",
+  };
+  const key = `${process.platform}-${process.arch}`;
+  const target = platformMap[key];
+  if (!target) {
+    throw new Error(
+      `package-runtime: no workerd platform package mapped for ${key}. ` +
+        "Add it to the platform map in apps/host-selfhost/scripts/package-runtime.ts.",
+    );
+  }
+  return `@cloudflare/${target}`;
+};
+
 const externalPackages = [
   "@cloudflare/worker-bundler",
   "quickjs-emscripten",
@@ -46,6 +70,7 @@ const externalPackages = [
   "@jitl/quickjs-wasmfile-release-asyncify",
   "@jitl/quickjs-wasmfile-debug-asyncify",
   "workerd",
+  workerdPlatformPackage(),
   libsqlNativePackage(),
 ] as const;