chore: document architecture gaps and add dev hot reload

- add npm dev:hot wrapper using cargo-watch and page reload polling

- add mnote-web dev hot reload endpoint and coverage

- record current architecture review and tracked bug findings across realtime, tree, editor, and AI runtimes

Verification:

- node scripts/task-dev-hot-plan-test.js

- node --check scripts/dev-hot.js

- cargo test -p mnote-web dev_hot -- --nocapture

scripts/dev-hot.js

@@ -0,0 +1,72 @@
+#!/usr/bin/env node
+"use strict";
+
+/**
+ * dev:hot 启动入口。
+ *
+ * 目标：
+ * - 不改动 desktop:hot 的默认行为。
+ * - 使用 cargo-watch 自动重编译并重启 mnote-web。
+ * - 通过 MNOTE_WEB_DEV_HOT_RELOAD 启用页面端轻量 reload 轮询。
+ */
+
+const { spawn } = require("node:child_process");
+const fs = require("node:fs");
+const path = require("node:path");
+
+const rootDir = path.resolve(__dirname, "..");
+const desktopHotPath = path.join(rootDir, "scripts", "desktop-hot.js");
+
+function cargoWatchCommand(env = process.env) {
+  const extraArgs = String(env.MNOTE_WEB_DEV_HOT_CARGO_WATCH_ARGS || "").trim();
+  const watchArgs = [
+    "-w crates/mnote-web/src",
+    "-w crates/mnote-web/Cargo.toml",
+    "-w Cargo.toml",
+    "-x \"run -p mnote-web --bin mnote-web\"",
+  ];
+  if (extraArgs) watchArgs.unshift(extraArgs);
+  return `cargo watch ${watchArgs.join(" ")}`;
+}
+
+function buildDevHotEnv(baseEnv = process.env) {
+  return {
+    ...baseEnv,
+    MNOTE_WEB_DEV_HOT_RELOAD: "1",
+    MNOTE_WEB_CMD: String(baseEnv.MNOTE_WEB_CMD || "").trim() || cargoWatchCommand(baseEnv),
+  };
+}
+
+function main() {
+  if (!fs.existsSync(desktopHotPath)) {
+    throw new Error(`缺少 desktop-hot 入口：${desktopHotPath}`);
+  }
+
+  const child = spawn(process.execPath, [desktopHotPath], {
+    cwd: rootDir,
+    env: buildDevHotEnv(process.env),
+    stdio: "inherit",
+  });
+
+  child.on("exit", (code, signal) => {
+    if (signal) {
+      process.kill(process.pid, signal);
+      return;
+    }
+    process.exit(code ?? 0);
+  });
+
+  child.on("error", (error) => {
+    console.error(`[dev-hot] 启动失败：${error.message}`);
+    process.exit(1);
+  });
+}
+
+if (require.main === module) {
+  main();
+}
+
+module.exports = {
+  buildDevHotEnv,
+  cargoWatchCommand,
+};