nodejs: lazy per-backend libsimplex download (#7543)

* ci: drop dlltool from nodejs libs release

* nodejs: download libsimplex into the user cache

* nodejs: load libsimplex at runtime per backend

* support bot: select db backend with --db

* python: match backend switch error text

* nodejs: download prebuilt add-on at runtime

* ci: build nodejs add-on for libs release

* calculator bot: load libs and add-on in image

* support bot: document add-on override

* nodejs: keep worker test output free of colors

* nodejs: verify sha-256 of downloaded assets

* python: pin core libs release with sha-256

* ci: add packages release script

* ci: release nodejs add-on and open release PR

* ci: publish packages after release PR

* nodejs: keep npm from building the add-on

* calculator bot: build the add-on in the image

* ci: merge package release workflows

* ci: open release PR when publishing fails

* ci: build the add-on with node-addon-api only

* ci: skip publishing in a called packages workflow

* ci: update package release actions to Node 24

* nodejs: fix receive test timeout

* ci: test packages on pull requests

* nodejs: reuse the database in the close stress test

* ci: keep failed package tests from hanging

* ci: skip preset server tests

* ci: plan package builds from the changes

* ci: test packages with a given libsimplex

* ci: test packages with libsimplex of core changes

packages/simplex-chat-nodejs/src/core.ts

@@ -1,6 +1,28 @@
 import {ChatEvent, ChatResponse, T} from "@simplex-chat/types"
+import * as libs from "./libs"
 import * as simplex from "./simplex"
 
+export type Backend = "sqlite" | "postgres"
+
+let loading: {backend: Backend, promise: Promise<void>} | undefined
+
+/**
+ * Resolve (downloading on first use) and load libsimplex for the backend.
+ * One libsimplex per process: the Haskell runtime is initialized once, and another backend is rejected.
+ */
+export function loadLibrary(backend: Backend): Promise<void> {
+  if (loading) {
+    if (loading.backend === backend) return loading.promise
+    return Promise.reject(new Error(`libsimplex already loaded with backend=${loading.backend}; cannot switch to ${backend} in the same process`))
+  }
+  const promise = Promise.all([libs.resolveLibsDir(backend), libs.resolveAddonPath()])
+    .then(([dir, addon]) => simplex.load(addon, libs.libPath(dir)))
+  loading = {backend, promise}
+  // a failed download or load can be retried
+  promise.catch(() => { loading = undefined })
+  return promise
+}
+
 /**
  * Initialize chat controller
  * @param {number} [queueSize] - Size of internal queues, the core default is used when omitted.