# UnifiedWriter dev-5.67.2 Demo Local Disk Save

## Purpose

This development build updates the demo Host callbacks so that Demo file save follows the single-file processing policy v1.3.
The package runtime does not choose a storage provider.  The Demo acts as a sample Host and implements `OpenFile` / `SaveFile` callbacks.

## Behavior

- `OpenFile(request, context)` uses the browser native local file picker through `showOpenFilePicker()` when available.
- `SaveFile(payload, context)` writes `HostSavePayload/v1.payload.data` to a user-selected Local Disk file through `showSaveFilePicker()` and `FileSystemWritableFileStream` when available.
- Save As does not use the old in-page filename-only dialog in browsers that support the File System Access API.
- Save reuses the last writable file handle when available.  If no handle is available, the Demo asks the user to choose a Local Disk file destination.
- Download/FileSaver is fallback only for browsers that do not support File System Access API.

## Affected demos

- `demo/basic.html`
- `demo/parameters.html`
- `demo/callback-io.html`

## Policy alignment

The direction is now clearly separated:

1. Demo/Workbench calls `OpenFile` to ask the Host side to obtain a file.
2. Host returns `HostFilePayload/v1` to the Control side.
3. EditorControl interprets the payload through `openFile` or command routing.
4. EditorControl generates `HostSavePayload/v1` with `payload.data` for save/export.
5. Demo/Host `SaveFile` writes `payload.data` to the selected Local Disk destination.