P0.3: add iOS @Environment(\.dataManager) key

Introduces DataManagerEnvironmentKey + EnvironmentValues.dataManager so
SwiftUI views can resolve DataManagerObservable via @Environment, mirroring
Compose's LocalDataManager ambient on the Kotlin side.

No view migrations yet — views continue to read DataManagerObservable.shared
directly. The actual screen-level substitution (fake DataManager for
parity-gallery / tests / previews) lands in P1 when ViewModels gain an
optional init param that accepts the environment-resolved observable. For
this commit we only need the key so P1 can wire against it.

Note: the iosSimulator Kotlin compile is broken at baseline (bb4cbd5)
with pre-existing "Unresolved reference 'testTagsAsResourceId'" errors
across 20+ screen files — Android-only semantics API imported in
commonMain. Swift-parse of the new file succeeds. Verified by checking
out bb4cbd5 and rerunning ./gradlew :composeApp:compileKotlinIosSimulatorArm64.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

iosApp/iosApp/Environment/DataManagerEnvironment.swift

@@ -0,0 +1,31 @@
+import SwiftUI
+import ComposeApp
+
+/// SwiftUI `@Environment` mirror of Compose's `LocalDataManager` ambient.
+///
+/// Production views resolve `@Environment(\.dataManager)` to
+/// `DataManagerObservable.shared` — the existing Swift-side mirror of the
+/// shared Kotlin `DataManager` singleton. Tests, previews, and the
+/// parity-gallery override via:
+///
+/// ```swift
+/// MyView()
+///     .environment(\.dataManager, FakeDataManagerObservable())
+/// ```
+///
+/// This key is intentionally introduced without migrating any SwiftUI
+/// screens yet. Views continue to access `DataManagerObservable.shared`
+/// directly; the actual view-level migration lands when parity-gallery
+/// fixtures arrive (P1), at which point ViewModels will gain an optional
+/// init param that accepts a `DataManagerObservable` resolved from this
+/// environment key.
+struct DataManagerEnvironmentKey: EnvironmentKey {
+    static let defaultValue: DataManagerObservable = DataManagerObservable.shared
+}
+
+extension EnvironmentValues {
+    var dataManager: DataManagerObservable {
+        get { self[DataManagerEnvironmentKey.self] }
+        set { self[DataManagerEnvironmentKey.self] = newValue }
+    }
+}