Record WooCommerce purchase revenue; stop counting checkouts as revenue

[onamfc]

Summary

LinkForty sums a conversion's value as revenue. The pixel's track(name, properties) reads the amount from properties.value only.

• purchase now sends the order total as value. revenue is still sent with the same total, so a linkforty_purchase_properties filter written against 0.1.0 keeps working.
• begin_checkout sends the cart total as cart_total instead of value. As value it was summed as revenue: every checkout page view added its cart total. This renames a documented property, so it's listed as breaking in the changelog (0.x).
• add_to_cart is unchanged (it sends price, which isn't summed).
• The LinkForty_Events::track docblock, README, readme.txt and the event contract in CONTRIBUTING.md now say value is revenue. CONTRIBUTING also said the pixel flattens nested objects; it drops them.

Test plan

• composer lint — no syntax errors
• composer phpcs — clean
• On a test store: place an order through a LinkForty link; the pixel's purchase request carries "value": <order total> and the dashboard's Web tab shows it as conversion value
• Visit checkout with a non-empty cart; begin_checkout carries cart_total and no value