Running two ng serve instances (same project, different configurations) causes “504 Outdated Optimize Dep” errors in Angular CLI 20.3.8

### Command

serve

### Is this a regression?

- [x] Yes, this behavior used to work in the previous version

### The previous version in which this bug was not present was

20.3.6

### Description

Since updating to Angular v20.3.9 / CLI v20.3.8, it’s no longer possible to run two ng serve instances for the same project (e.g. localized builds) in parallel.
The second instance fails to load with a 504 (Outdated Optimize Dep) error for one or more chunks (e.g. chunk-5V7QYGNA.js).

This setup worked correctly in previous Angular 20.3.6 versions (before the CLI migrated the dev-server to the Vite-based implementation).

The issue appears to be caused by both ng serve processes sharing the same Vite optimizer cache directory inside .angular/cache/.../vite.
When one process updates or invalidates the cache, the other process detects stale metadata and refuses to serve pre-bundled chunks.

### Minimal Reproduction

package.json scripts
"scripts": {
"start": "ng serve --host 0.0.0.0",
"startfr": "ng serve -c fr --host 0.0.0.0",
"startfr-alt": "ng serve -c fr --host 0.0.0.0 --port 4201",
"start:both": "concurrently \"npm run start\" \"npm run startfr-alt\""
}

Steps to reproduce
1. Run npm run start:both.
2. Wait for both dev servers to start.
3. Observe that the second instance fails to load in the browser console with:
Failed to load resource: the server responded with a status of 504 (Outdated Optimize Dep)

### Exception or Error

```text
Failed to load resource: the server responded with a status of 504 (Outdated Optimize Dep)
```

### Your Environment

```text
Angular CLI: 20.3.8
Node: 24.1.0
Package Manager: yarn 4.9.2
OS: darwin arm64

Angular: 20.3.9
... animations, common, compiler, compiler-cli, core, forms
... language-service, localize, platform-browser
... platform-browser-dynamic, router

Package Version
---------------------------------------
@angular-devkit/architect 0.2003.8
@angular-devkit/build-angular 20.3.8
@angular-devkit/core 20.3.8
@angular-devkit/schematics 20.3.8
@angular/cdk 20.2.11
@angular/cli 20.3.8
@angular/material 20.2.11
@schematics/angular 20.3.8
rxjs 7.8.2
typescript 5.8.3
zone.js 0.15.1
```

### Anything else relevant?

_No response_

package.json start:both の再現手順から始め、両方の ng serve プロセスを実行しながら、共有されている .angular/cache/.../vite オプティマイザーディレクトリを調べます。異なる構成の並列サーバーが 504 Outdated Optimize Dep エラーなしで正常に読み込まれれば、修正は完了です。

索引モデルが issue の本文から書いたものです。