Initial release: HDWebmobile Buy Now Button

A direct-to-checkout Buy Now button on simple products. Motivated by CWE-20
(Improper Input Validation) in several competing "Buy Now"/"Direct
Checkout" plugins: the skip-the-cart link carried the quantity (and, in one
case, a price override) directly in its own URL query string and used it
verbatim when adding the item to the cart, letting an edited link request
an unintended quantity or manipulate the price charged.

Closed by construction: the quantity added to the cart is always the
literal number 1, written directly into the code -- there is no variable
anywhere in the request handling that could carry a different value in
from a link. The price is always whatever WooCommerce's own cart lookup
finds for the real product at that moment. A variation id (for a variable
product) is only ever accepted if it genuinely belongs to the product it's
claimed for.

Verified via a 17-case PHP unit suite -- including a variation id claimed
against the wrong parent product being rejected, out-of-stock/nonexistent
products resolving to nothing, reflection confirming resolve_product() has
no quantity/price parameter at all, a source-level check that no $_GET/
$_POST quantity or price value is ever read anywhere in the plugin, and a
real WooCommerce cart proving the resolved product is added at exactly its
own real price. Plugin Check clean.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
