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/commands/start/index.ts

@@ -1,19 +1,24 @@
 import { Command } from "commander";
+import {
+    DistDirOptionDescription,
+    DistDirOptionFlags,
+} from "../dist-dir-option";
 import path from "path";
 import type { BunSpawnOptions } from "../../types";
 import writeErrorFile from "../../functions/write-error-file";
 import grabDirNames from "../../utils/grab-dir-names";
 import { existsSync, rmSync } from "fs";
 
-const { BUNX_BUNDLER_ERROR_EXIT_FILE } = grabDirNames();
-
 const MAX_RETRIES = 5;
 const RETRY_RESET_AFTER_MS = 10000;
 
 export default function () {
     return new Command("start")
         .description("Start production server")
+        .option(DistDirOptionFlags, DistDirOptionDescription)
         .action(async () => {
+            const { BUNX_BUNDLER_ERROR_EXIT_FILE } = grabDirNames();
+
             // A stale error file from a previous session makes the bundler
             // run in "silent" mode and pages fail with misleading messages.
             rmSync(BUNX_BUNDLER_ERROR_EXIT_FILE, { force: true });