Adds JSDoc documentation for the `memfs` export from `rolldown/experimental` to help users understand the in-memory filesystem API for browser builds.

### Changes
- Added comprehensive JSDoc to `memfs` in `experimental-index.ts`:
  - Description of `fs` (Node.js-compatible filesystem API) and `volume` (underlying Volume instance)
  - Note that it returns `undefined` in Node.js builds
  - Link to upstream [memfs](https://github.com/streamich/memfs) library

### Example
```typescript
import { memfs } from 'rolldown/experimental';

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

// Read files from the virtual filesystem
const content = memfs?.fs.readFileSync('/src/index.js', 'utf8');
```

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<details>

<summary>Original prompt</summary>

> Solve #7515

</details>

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## [1.0.0-beta.56] - 2025-12-22

### 💥 BREAKING CHANGES

- rename `MIXED_EXPORT` error to `MIXED_EXPORTS` (#7565) by @sapphi-red

### 🚀 Features

- rename `id` property to `exporter` in CIRCULAR_REEXPORT error (#7592) by @sapphi-red
- add `ids` property to `CIRCULAR_DEPENDENCY` error (#7591) by @sapphi-red
- node/dev: expose `devMode.lazy` (#7549) by @hyf0
- set log and pos properties for `parseAst` function errors (#7568) by @sapphi-red
- set log and pos properties for logs (#7567) by @sapphi-red
- test-dev-sever: support to manually configure port, run tests in concurrent (#7576) by @hyf0
- add `exporter` property to `MISSING_EXPORT` error (#7564) by @sapphi-red
- add `id` property to `PARSE_ERROR` error (#7563) by @sapphi-red
- support ImporterId hook filter (#7540) by @IWANABETHATGUY

### 🐛 Bug Fixes

- types: better "go to definition" experience for interface `OutputPlugin` (#7610) by @KazariEX
- `postBanner` content should be placed after shebang (#7583) by @btea
- use sanitized filename for preserve modules chunk name (#7603) by @IWANABETHATGUY
- correct filter out unused cjs namespace  (#7602) by @IWANABETHATGUY
- watch: property respect `notify.pollInternal` and `notify.compareContents` (#7595) by @sapphi-red
- make `cleanDir` work with default output directory (#7579) by @shulaoda
- merge `MISSING_NAME_OPTION_FOR_UMD_EXPORT` error to `MISSING_NAME_OPTION_FOR_IIFE_EXPORT` error (#7566) by @sapphi-red
- dev/hmr: ensure cjs modules with no exports reference correct `module` identifier (#7544) by @leegeunhyeok

### 🚜 Refactor

- remove `stable_id` field from `PARSE_ERROR` error (#7593) by @sapphi-red
- make include_runtime_symbol reuseable after linking stage (#7580) by @IWANABETHATGUY
- rust/dev: construct the bundler within itself (#7553) by @hyf0
- rust/watcher: polish API of `Watcher` struct (#7551) by @hyf0
- use `LinkingMetadata::stmt_info_included` to check if a stmt_info is included (#7572) by @IWANABETHATGUY
- use `LinkingMetadata::is_included` to check if a module is included (#7571) by @IWANABETHATGUY
- store module and stmt_info is included info to module meta (#7570) by @IWANABETHATGUY
- make include_* method reunsable after linking stage (#7552) by @IWANABETHATGUY
- rust/watcher: construct the bundler within watcher itself (#7550) by @hyf0
- extract make include_runtime_symbol reusable (#7546) by @IWANABETHATGUY

### ⚙️ Miscellaneous Tasks

- renovate: add `kill-port` in `ignoreDeps` in renovate.json (#7619) by @sapphi-red
- deps: update rust crates (#7617) by @renovate[bot]
- deps: update npm packages (#7616) by @renovate[bot]
- deps: update github-actions (#7615) by @renovate[bot]
- ci: skip benchmark workflows on draft PRs (#7611) by @Copilot
- deps: update dependency rolldown-plugin-dts to ^0.19.0 (#7607) by @renovate[bot]
- deps: update dependency oxlint-tsgolint to v0.10.0 (#7601) by @renovate[bot]
- deps: update dependency rolldown-plugin-dts to v0.18.4 (#7599) by @renovate[bot]
- deps: update notify (#7594) by @sapphi-red
- test-dev-server: add retry mechanism to hmr-full-bundle-mode tests (#7588) by @Copilot
- deps: update napi to v3.7.1 (#7590) by @renovate[bot]
- add JSDoc documentation for memfs type (#7587) by @Copilot
- deps: update dependency oxlint to v1.34.0 (#7589) by @renovate[bot]
- move some tests in ignored-by-unsupported-features that are passing (#7569) by @sapphi-red
- deps: update dependency oxlint-tsgolint to v0.9.2 (#7582) by @renovate[bot]
- deps: update oxc resolver to v11.16.0 (#7574) by @renovate[bot]
- test/dev-server: don't run `pnpm install` during tests (#7560) by @hyf0
- test/dev-server: use `kill-port@1` to improve performance (#7575) by @hyf0
- normalize error object to make some Rollup tests pass (#7562) by @sapphi-red
- ci: separate dev-server(hmr) tests and normal tests (#7558) by @hyf0
- ci: make native rolldown build reusable (#7557) by @hyf0
- resolve some TODOs (#7561) by @sapphi-red

### ❤️ New Contributors

* @KazariEX made their first contribution in [#7610](#7610)
* @leegeunhyeok made their first contribution in [#7544](#7544)

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