`PurchaseResult`. Surfaced when the first iOS dogfood project pulled
v1.4.1 via SwiftPM and Xcode rejected the SDK source with two
cascading errors in `syncPurchases(...)`:

- `Value of type 'PurchaseResult' has no member 'idempotent_replay'`
- `Cannot convert value of type 'Any' to expected dictionary value
  type 'String'` (cascade — the Swift type-checker gives up on
  `[String: Any]` inference once a member resolves unknown)

**Fixed:**

- `PurchaseResult` gains `public let idempotentReplay: Bool?` with
  a `CodingKey` mapping the Swift-idiomatic camelCase property to
  the wire's `idempotent_replay` (snake_case, set by the backend's
  idempotency-response-cache middleware on cache-hit retries per
  the `idempotency-key-deterministic` contract).
- `syncPurchases(...)` now reads `result.idempotentReplay`. The
  analytics event property key stays `idempotent_replay` on the
  wire so dashboards joining `purchase.completed` across SDKs see
  the same key Web/Node/RN/Android already emit.

No new API surface; no behavioural change beyond the typed access
unlocking what the runtime was already receiving. Bundled
contracts (`Resources/contracts.json`) regenerated so `bundledIn`
stamps `@cross-deck/swift@1.4.2`.