pastes and runs end-to-end without the developer having to
hand-roll any wiring.

**Added:**

- **`EnvironmentValues.crossdeck`** — public SwiftUI environment key
  shipped by the SDK. Consumers can now write
  `ContentView().environment(\.crossdeck, cd)` at the App root and
  `@Environment(\.crossdeck) private var cd` inside any view, with
  zero `EnvironmentKey` boilerplate on the consumer side. Returns
  `Crossdeck?` so the optional-chain pattern at every call site
  keeps the host app crash-proof when the SDK didn't start. Lives
  under `#if canImport(SwiftUI)` so non-Apple-platform resolves
  still compile.
- **`CrossdeckEnvironment`** — public typealias for the SDK's
  `Environment` enum. Avoids the symbol collision every SwiftUI
  consumer hits the moment they `import Crossdeck` alongside
  `import SwiftUI` (SwiftUI's own `Environment` property wrapper).
  The bare `Environment` name stays exported for back-compat and
  keeps working unqualified in files that don't import SwiftUI.

**Changed:**

- Quickstart snippet (dashboard SDKs page → iOS) now uses
  `CrossdeckEnvironment` in place of bare `Environment`, so the
  copy-pasted boot code compiles cleanly inside a `@main App`
  file that also imports SwiftUI.

No behavioural change; both additions are purely ergonomic surface
that closes the "first dogfood paste-and-run" friction loop.