feat: configurable multi-format download split button on embeds

[erseco]

Closes #32.

Summary

Adds a per-embed split-button next to embedded eXeLearning content that lets visitors download the resource in any of the formats produced by the editor's SharedExporters bundle, picked client-side. The .elpx source link continues to work as before; the new formats are produced by lazy-loading the static editor in a hidden iframe and invoking SharedExporters.quickExport via a new WP_REQUEST_EXPORT protocol message.

Format	ID	Output filename
HTML5 web	`html5`	`_web.zip`
SCORM 1.2	`scorm12`	`_scorm.zip`
IMS CP	`ims`	`_ims.zip`
EPUB 3	`epub3`	`.epub`
eXeLearning source	`elpx`	`.elpx`

SCORM 2004 and HTML5 single-page are intentionally excluded per the requirements.

What changed

• Block UI (`assets/js/elp-upload.js`) — new `InspectorControls` panel "Download options" with a Show download button toggle and a checkbox per format. New block attributes: `showDownload` (bool) and `downloadFormats` (string[]). Defaults: all formats enabled.
• Shortcode (`public/class-shortcodes.php`) — new `show_download` and `download_formats` (comma-separated) attributes that mirror the block.
• Frontend rendering — both the block and the shortcode now emit the split-button via the new `ExeLearning_Download_Button_Renderer`. When only one format is enabled, the button collapses to a plain link; when none are enabled, the toolbar omits it.
• Download orchestrator (`assets/js/wp-exe-download.js`) — handles clicks, lazy-creates a hidden iframe pointed at the new export bootstrap, sends `WP_REQUEST_EXPORT` postMessages, receives bytes, triggers the browser download. The iframe is reused for additional downloads and disposed after a 60s idle period.
• Public export bootstrap (`includes/class-export-bootstrap.php`) — anonymous `template_redirect` handler that serves a stripped-down version of the static editor for export-only use. No save endpoint, no user data, no CSRF token; reuses the same dist/static/index.html template and the existing bridge.
• Bridge (`assets/js/wp-exe-bridge.js`) — extended with a `WP_REQUEST_EXPORT` handler that invokes `SharedExporters.quickExport` for the requested format and replies with `WP_EXPORT_FILE` (bytes + filename + mimeType).
• Format registry (`includes/class-download-formats.php`) — single source of truth for ids, labels, filename suffixes and MIME types.
• Styles (`assets/css/exelearning.css`) — split-button styling with dropdown menu, status text, busy state.

Sibling module

A parallel PR lands the same feature in `exelearning/omeka-s-exelearning` (also references this issue).

Test plan

• Insert an eXeLearning block in a post; sidebar shows "Download options" panel with all five formats checked.
• Save and view the post; toolbar shows the split-button with HTML5 web as the primary action and the four other formats in the dropdown.
• Click the primary action — receives a `_web.zip`.
• Open the dropdown, click each non-elpx entry — receives a file with the documented suffix and a valid format.
• Click the `.elpx` entry — downloads the raw attachment.
• Disable "Show download button" — toolbar disappears.
• Use the `[exelearning id="N" download_formats="elpx,epub3"]` shortcode — only those two appear in the toolbar.
• Verify `exporters.bundle.js` is only requested after the first non-elpx click, not on page load.
• Open the embed as a logged-out visitor — exports still work (anonymous bootstrap at `/?exe_export=1&attachment_id=N`).

[github-actions]

Test in WordPress Playground

Test the plugin with the code from this branch:

⚠️ The embedded eXeLearning editor is not included in this preview. You can install it from Settings > eXeLearning using the "Download & Install Editor" button. All other plugin features (ELP upload, shortcode, Gutenberg block, preview) work normally.