…7490)

Exports `builtinFs` from `rolldown/experimental` to allow users to access the virtual filesystem when using rolldown in browser environments.

### Changes

- Export `builtinFs: { fs, volume }` from `rolldown/experimental`
  - `fs`: The filesystem instance from the `memfs` package used by the WASM runtime
  - `volume`: The `Volume` instance from the `memfs` package used by the WASM runtime
  - Returns `undefined` in Node.js builds (only available in browser builds via `@rolldown/browser`)

### Usage

```typescript
import { builtinFs } from 'rolldown/experimental';

// Write files to virtual filesystem before bundling
builtinFs?.volume.fromJSON({
  '/src/index.js': 'export const foo = 42;',
  '/package.json': '{"name": "my-app"}'
});
```

<!-- START COPILOT ORIGINAL PROMPT -->

<details>

<summary>Original prompt</summary>

>
> ----
>
> *This section details on the original issue you should resolve*
>
> <issue_title>[Feature Request]: export `__volume` in rolldown browser to allow building complex packages</issue_title>
> <issue_description>### What problem does this feature solve?
>
> Hi,
>
> We are trying to use rolldown in the browser to bundle files that depend on node_modules downloaded packages. For that to work, we need to have direct access to __volume. See #6218 (comment)
>
> ### What does the proposed API look like?
>
> Simply export `__volume` so that it can be accessed directly.</issue_description>
>
> <agent_instructions>Only add the export under `rolldown/experimental`</agent_instructions>
>
> ## Comments on the Issue (you are @copilot in this section)
>
> <comments>
> </comments>
>

</details>

<!-- START COPILOT CODING AGENT SUFFIX -->

- Fixes #7489

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---

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## [1.0.0-beta.55] - 2025-12-17

### 🚀 Features

- add validation errors for incompatible `inlineDynamicImports` options (#7539) by @Copilot
- rolldown_plugin_vite_reporter: truncate long importer lists in ineffective dynamic import warnings (#7528) by @Copilot
- export `memfs` from `rolldown/experimental` for browser builds (#7490) by @Copilot
- implement `postBanner` and `postFooter` (#7487) by @sevenc-nanashi
- port getLogFilter helper from Rollup for advanced log filtering (#6890) by @taearls
- dev: initialize `rolldown_plugin_lazy_compilation` (#7488) by @hyf0
- enable `experimental.transform_hires_sourcemap: 'boundary'` by default (#7478) by @sapphi-red
- apply merge CJS namespace optimizations more generally (#7475) by @sapphi-red

### 🐛 Bug Fixes

- avoid panic on invalid values for `output.exports`, `output.format`, `output.hash_characters` (#7542) by @sapphi-red
- handle `__proto__` export for module namespace correctly (#7534) by @sapphi-red
- handle `__proto__` export correctly (#7533) by @sapphi-red
- rolldown_plugin_vite_resolve: acquire resolver lock before package json resolution (#7524) by @sapphi-red
- disable syntax transform optimization for `minify: 'dce-only'` (#7520) by @sapphi-red
- arrow function expression keep names (#7519) by @IWANABETHATGUY
- skip symbols that are imported from other module when deconflicting module symbols (#7510) by @IWANABETHATGUY
- preserve class names in assignment expressions with keepNames option (#7491) by @IWANABETHATGUY
- accessor with decorators should be kept (#7499) by @Copilot
- rolldown_plugin_vite_reporter: support `Infinity` for chunk_limit (#7497) by @shulaoda
- change filename template validation errors from UNHANDLEABLE_ERROR to INVALID_OPTION (#7472) by @Copilot
- `no entry found for key` error when merging CJS namespace exports (#7474) by @sapphi-red
- generate output with syntax error when wrapped esm module using tla syntax (#7468) by @IWANABETHATGUY

### 🚜 Refactor

- dev: rename `experimental.hmr` to `experimental.devMode` (#7527) by @hyf0
- pass addon option to rust as it is (#7526) by @hyf0
- remove unused stmtinfo meta flag (#7518) by @IWANABETHATGUY

### 📚 Documentation

- clarify `closeBundle` behavior (#7525) by @sapphi-red

### 🧪 Testing

- add tests for static string postBanner and postFooter (#7516) by @Copilot
- disable pluginTimings in Rust integration tests to avoid snapshot noise (#7485) by @shulaoda
- port Rollup JSX tests to `crates/rolldown/tests/rollup` (#7480) by @sapphi-red
- re-triage some esbuild JSX preserve tests (#7479) by @sapphi-red
- disable pluginTimings by default to avoid snapshot noise (#7471) by @shulaoda

### ⚙️ Miscellaneous Tasks

- mark feature that will not be supported as ignored in rollup test status (#7535) by @sapphi-red
- support `banner`/`footer`/`intro`/`outro` for config variants (#7532) by @sapphi-red
- `dce/dce_of_decorators` esbuild test is now passing (#7531) by @sapphi-red
- deps: update github-actions (major) (#7495) by @renovate[bot]
- deps: update npm packages (#7493) by @renovate[bot]
- add a check to verify failedReasons are updated (#7521) by @sapphi-red
- deps: update test262 submodule for tests (#7523) by @sapphi-red
- deps: update taiki-e/install-action action to v2.63.2 (#7496) by @renovate[bot]
- deps: update oxc apps (#7511) by @renovate[bot]
- deps: update oxc to v0.103.0 (#7513) by @camc314
- fix format in `.github/instructions/bug-investigation.instructions.md` (#7506) by @sapphi-red
- add instructions for REPL decoding (#7502) by @sapphi-red
- deps: update github-actions (#7492) by @renovate[bot]
- deps: cargo-shear v1.9.0 (#7483) by @Boshen
- deps: update dependency oxlint-tsgolint to v0.9.0 (#7484) by @renovate[bot]
- deps: update dependency oxlint-tsgolint to v0.8.6 (#7470) by @renovate[bot]
- add syntax validation for test output chunks that are not executed (#7466) by @IWANABETHATGUY
- deps: update dependency rust to v1.92.0 (#7467) by @renovate[bot]
- deps: update test262 submodule for tests (#7457) by @sapphi-red

### ❤️ New Contributors

* @sevenc-nanashi made their first contribution in [#7487](#7487)
* @taearls made their first contribution in [#6890](#6890)

Co-authored-by: shulaoda <[email protected]>