refactor(vite)!: make @vue-tui/vite dev-only; build apps with tsdown (#247)

`vite build` is a browser-first bundler, so producing a runnable Node program
from it meant constantly overriding its defaults (platform, externalization,
inlineDynamicImports, module-preload). Building a Node app is a Node bundler's
job — so split vue-tui's two jobs across two tools:

- @vue-tui/vite is now DEV-ONLY (in-terminal dev server + HMR). Removed
  buildConfigPlugin and the externalize predicate: deleted src/build.ts,
  src/external.ts (+ external.spec.ts) and test/build.sequential.test.ts.
  vueTui() returns just the dev + dev-vmod plugins, and entry normalization is
  dev-only; dev.spec.ts updated.
- Production builds move to tsdown + unplugin-vue: a plain tsdown.config.ts that
  bundles the whole app into one self-contained Node file (dist/*.mjs) runnable
  with no node_modules. platform:node keeps builtins external and emits a real
  createRequire for CJS deps; deps.alwaysBundle inlines everything else.
- Migrated all five examples to tsdown (including flappy-bird, which hand-rolled
  a vite lib-mode self-contained build), and rewrote the examples smoke suite to
  build via tsdown and launch each self-contained bundle from an empty sandbox.
- Added tsdown + unplugin-vue-jsx to the catalog; updated the root and
  @vue-tui/vite READMEs.

BREAKING CHANGE: @vue-tui/vite no longer configures `vite build`. Build the app
with tsdown instead (see the @vue-tui/vite README, examples/*, and the starter).

examples/flappy-bird/tsdown.config.ts

@@ -0,0 +1,12 @@
+import { defineConfig } from "tsdown";
+import Vue from "unplugin-vue/rolldown";
+
+// Self-contained Node build → dist/game.mjs, runnable with no node_modules present. See
+// examples/basic-template/tsdown.config.ts for the platform:node + deps.alwaysBundle rationale.
+export default defineConfig({
+  entry: { game: "src/main.ts" },
+  platform: "node",
+  format: "esm",
+  deps: { alwaysBundle: [/./], onlyBundle: false },
+  plugins: [Vue()],
+});