Add support for Twitter Card image/title/description overrides on posts

### Description

As seen in #161, this issue covers adding per-post override fields for Twitter Card metadata (image, title, description), following the same pattern already established for Open Graph tags in #124 (implemented on the `v2.0` branch via `src/features/class-open-graph.php`).

### Acceptance Criteria

- Add per-post fields for Twitter Card title, description, and image, using the same sidebar UI location as the Open Graph fields (see #141).
- Register the fields as post meta via `register_meta_helper()` so they're exposed in the REST API, consistent with the Open Graph fields.
- Fall back in this order when a Twitter field is not explicitly set: Twitter field → corresponding Open Graph field (which itself falls back to post title / excerpt / featured image) → nothing.
- Render `twitter:card`, `twitter:title`, `twitter:description`, and `twitter:image` tags in `wp_head` on singular posts, gated by post type support (mirroring `add_post_type_support( $post_type, 'open-graph' )`).
- Add tests covering the override and fallback chain.

### Use Case

Customizing post meta details such as Twitter Card image, title, and description so links shared on Twitter/X display the intended preview.

### Note

This depends on the Open Graph feature currently living on the `v2.0` branch, not `main`. Confirm which branch this should target before implementation.

First confirm the target branch, since the Open Graph implementation is on v2.0 rather than main. Read src/features/class-open-graph.php and the sidebar UI referenced by #141, then trace register_meta_helper() and the wp_head output; done means the three overrides, fallback chain, supported-post-type rendering, and accompanying tests are covered.

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