Rating shortcode: stable identifier fallback when no current post

Follow-up to #104.

In the rating-embed branch of `polldaddy-shortcode.php`, `$unique_id` and `$item_id` are derived from the current post:

```php
if ( empty( $unique_id ) && $post ) {
$unique_id = is_page() ? 'wp-page-' . $post->ID : 'wp-post-' . $post->ID;
}
if ( empty( $item_id ) && $post ) {
$item_id = is_page() ? '_page_' . $post->ID : '_post_' . $post->ID;
}
```

When there is no current post (taxonomy archives, etc.) and the attributes aren't supplied in the shortcode, both stay empty. Downstream that yields `id="pd_rating_holder_{$rating}"` and `PDRTJS_settings_{$rating}` with no post component, so multiple ratings sharing the same `rating` id on the same post-less page collide on identical element IDs, and `unique_id` is empty in the settings JSON.

This collision predates #104 (the previous code produced a constant `_post_` suffix in the same situation) — #104 just removed the warning. Tracking the underlying behaviour separately.

### Proposed change
Fall back to a deterministic value when `$post` is absent, e.g. `get_queried_object_id()` or the `rating` id, so element IDs stay unique. If an empty `unique_id` is acceptable per the rating JS contract, add a short comment saying so instead.

### Tests
The shortcode currently has no coverage. Add PHPUnit tests:
- No current post: `[polldaddy rating=5]` renders markup without notices.
- Current post set: identifiers still resolve to `wp-post-{ID}` / `_post_{ID}`.
- Two ratings with the same `rating` id on a post-less page produce distinct element IDs.

Start in the rating-embed branch of polldaddy-shortcode.php and trace how $unique_id and $item_id are derived when no current post exists. Check the rating JavaScript contract, then add PHPUnit coverage for post-less and current-post rendering, including duplicate-rating identifiers; done means the tests pass without notices and verify stable, distinct markup IDs.

Written by the indexing model from the issue text.