debug extension / mode for content-mapper

@andrewbranch is already working on this.

Since Aug 27, 2026.

### 🔍 Search Terms

`content mapper`

`volar labs`
- only result is https://github.com/microsoft/TypeScript/issues/63800 -- which doesn't seem relevant

### ✅ Viability Checklist

- [x] This wouldn't be a breaking change in existing TypeScript/JavaScript code
- [x] This wouldn't change the runtime behavior of existing JavaScript code
- [x] This could be implemented without emitting different JS based on the types of the expressions
- [x] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
- [x] This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
- [x] This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

### ⭐ Suggestion

Volar has a really helpful tool for debugging mapping, here: https://volarjs.dev/core-concepts/volar-labs/

it would be great to have something similar for content-mappers

### 📃 Motivating Example

In a big project at work, I'm not seeing completions / hovers in VSCode, even though I have them in neovim and in smaller projects in vscode. the more debugging utilities available, the more I'd be able to narrow what the problem could be (config setup or otherwise)

### 💻 Use Cases

see above