doc: add VFS section to single-executable-applications

Document how to use the Virtual File System (VFS) with Single
Executable Applications to access bundled assets through standard
fs APIs. Includes usage of fs.getSeaVfs(), fs.hasSeaAssets(),
supported operations, and the createRequire() requirement for
loading bundled modules.

Author: mcollina

doc/api/single-executable-applications.md

@@ -238,6 +238,90 @@ const raw = getRawAsset('a.jpg');
 See documentation of the [`sea.getAsset()`][], [`sea.getAssetAsBlob()`][],
 [`sea.getRawAsset()`][] and [`sea.getAssetKeys()`][] APIs for more information.
 
+### Virtual File System (VFS) for assets
+
+> Stability: 1 - Experimental
+
+Instead of using the `node:sea` API to access individual assets, you can use
+the Virtual File System (VFS) to access bundled assets through standard `fs`
+APIs. The VFS automatically populates itself with all assets defined in the
+SEA configuration and mounts them at a virtual path (default: `/sea`).
+
+To use the VFS with SEA:
+
+```cjs
+const fs = require('node:fs');
+
+// Check if SEA assets are available
+if (fs.hasSeaAssets()) {
+  // Initialize and mount the SEA VFS
+  const vfs = fs.getSeaVfs();
+
+  // Now you can use standard fs APIs to read bundled assets
+  const config = JSON.parse(fs.readFileSync('/sea/config.json', 'utf8'));
+  const data = fs.readFileSync('/sea/data/file.txt');
+
+  // Directory operations work too
+  const files = fs.readdirSync('/sea/assets');
+
+  // Check if a bundled file exists
+  if (fs.existsSync('/sea/optional.json')) {
+    // ...
+  }
+}
+```
+
+The VFS supports the following `fs` operations on bundled assets:
+
+* `readFileSync()` / `readFile()` / `promises.readFile()`
+* `statSync()` / `stat()` / `promises.stat()`
+* `lstatSync()` / `lstat()` / `promises.lstat()`
+* `readdirSync()` / `readdir()` / `promises.readdir()`
+* `existsSync()`
+* `realpathSync()` / `realpath()` / `promises.realpath()`
+* `accessSync()` / `access()` / `promises.access()`
+* `openSync()` / `open()` - for reading
+* `createReadStream()`
+
+#### Loading modules from VFS in SEA
+
+The default `require()` function in a SEA only supports loading Node.js
+built-in modules. To load JavaScript modules bundled as assets, you must use
+[`module.createRequire()`][]:
+
+```cjs
+const fs = require('node:fs');
+const { createRequire } = require('node:module');
+
+// Initialize VFS
+fs.getSeaVfs();
+
+// Create a require function that works with VFS
+const seaRequire = createRequire('/sea/');
+
+// Now you can require bundled modules
+const myModule = seaRequire('/sea/lib/mymodule.js');
+const utils = seaRequire('/sea/utils/helpers.js');
+```
+
+This is necessary because SEA uses a special embedder require that doesn't go
+through the standard module resolution hooks that VFS registers.
+
+#### Custom mount prefix
+
+By default, the VFS is mounted at `/sea`. You can specify a custom prefix
+when initializing the VFS:
+
+```cjs
+const vfs = fs.getSeaVfs({ prefix: '/app' });
+
+// Assets are now accessible under /app
+const config = fs.readFileSync('/app/config.json', 'utf8');
+```
+
+Note: `fs.getSeaVfs()` returns a singleton. The `prefix` option is only used
+on the first call; subsequent calls return the same cached instance.
+
 ### Startup snapshot support
 
 The `useSnapshot` field can be used to enable startup snapshot support. In this