fix for the SwiftUI symbol collision — closes friction #19 deeper
than v1.4.4 caught it.

**Why:** v1.4.4 added `typealias CrossdeckEnvironment = Environment`
to disambiguate consumer code that imports both `SwiftUI` and
`Crossdeck`. That helped consumers but left the module-level public
`Environment` enum in scope inside the SDK itself. When v1.4.5
shipped `View.crossdeckScreen("Name")`, the new `ViewModifier`
contained `@Environment(\.crossdeck) private var cd` — and the
compiler resolved `Environment` to the SDK's enum (closer than
SwiftUI's), failing with `'Environment' cannot be used as an
attribute`. The SDK author shipped a public type that collided
with the most-used SwiftUI property wrapper for their own code.

**Fix:** rename the source enum to `CrossdeckEnvironment` and
remove the typealias entirely. There is no short form — the
qualified name is the only public name. Inside the SDK module
`@Environment(...)` now resolves to SwiftUI's wrapper
unambiguously, and consumer code that already follows the docs
(`let environment: CrossdeckEnvironment = .production`) is
unchanged.

**Breaking:** consumer code using `Environment.production` /
`Environment.sandbox` must use `CrossdeckEnvironment.production` /
`CrossdeckEnvironment.sandbox`. Pre-launch, no shipping consumer
relies on the short name — all install snippets and dogfood code
already use the qualified form.