IPurchaseService (start/add-item/complete) + Purchase.AddItem/Complete
domain methods. PurchaseController implements a single-page recording
flow mirroring the proven ShoppingList UX pattern. Verified live
end-to-end via dotnet run + curl.

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

Purchase (aggregate root) and PurchaseItem per AGENTS.md §5.6.
AddItem composition + tightened PurchaseItem ctor deferred to
CW-STORY-05.4, mirroring the CW-STORY-03.1/03.3 split.

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