Make the dist directory configurable

Set it with `dist_dir` in bunext.config.ts or `--dist-dir` on any CLI
command (precedence: --dist-dir > BUNEXT_DIST_DIR > dist_dir > .bunext).
setupDistDir() resolves it in the CLI's preAction hook and at the start
of bunextInit(), so custom servers get the config value too.

- Read dist-derived paths from grabDirNames() inside functions instead of
  at import time; a guard test enforces this.
- Derive the client asset URL prefix from the dist dir instead of
  hard-coding /.bunext.
- Make the dev watcher ignore rules and probe follow the dist dir.
- Validate the dir: inside the project root, outside src/, public/ and
  node_modules/, URL-safe characters only.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/functions/server/bunext-req-handler.ts

@@ -10,6 +10,7 @@ import { AppData } from "../../data/app-data";
 import { log } from "../../utils/log";
 import fullRebuild from "./full-rebuild";
 import grabRouter from "../../utils/grab-router";
+import grabBunextPublicUrlPath from "../../utils/grab-bunext-public-url-path";
 import { NoSEORobotsValue } from "../../utils/disable-seo";
 import generateRobotsTxt from "../../utils/generate-robots-txt";
 
@@ -79,7 +80,7 @@ export default async function bunextRequestHandler({
             response = new Response(generateRobotsTxt({ disallow_all: true }), {
                 headers: { "Content-Type": "text/plain; charset=utf-8" },
             });
-        } else if (url.pathname.startsWith("/.bunext")) {
+        } else if (url.pathname.startsWith(`${grabBunextPublicUrlPath()}/`)) {
             response = await handleBunextPublicAssets({ req });
         } else if (url.pathname.startsWith("/api/")) {
             response = await handleRoutes({ req });