Introduce a first-class PurchaseCompleted lifecycle event so host apps can run
fulfillment (grant access, send receipts, provision licences) without coupling
to the ProductAction table or to a concrete purchasable model. It fires when a
purchase is created already-COMPLETED and when one transitions into COMPLETED,
and is transition-guarded so it does not re-fire on unrelated saves of an
already-completed purchase.
Also generalise the built-in ProductAction fulfillment in ProductPurchase:
the actionable product is now resolved via config('shop.models.product') /
'...product_price' (instead of a hard instanceof the bundled Product), and
callActions() is only invoked when the resolved product exposes it — so apps
overriding the models, or using IsSimplePurchasable host models, complete
cleanly. Existing behaviour for the bundled Product is unchanged.
Adds 4 EventsWiredUpTest cases; full suite 1404 green. Docs + README updated.