feat: add Apple Foundation Models on-device provider for iOS

[himattm]

Summary

• Adds a new :halogen-provider-apple-foundation KMP module (iOS-only targets) that wraps Apple's on-device FoundationModels framework (iOS 26+) behind the existing HalogenLlmProvider interface — the iOS counterpart to halogen-provider-nano on Android.
• Wires the new provider into sample-ios with auto-selection: Apple FM if SystemLanguageModel.default.availability == .available, else OpenAI (if key configured), else Demo. No UI changes.
• Keeps the iOS deployment target at 16 by weak-linking FoundationModels and gating Swift usage behind @available(iOS 26.0, *) + #if canImport(FoundationModels).

Design

FoundationModels is Swift-only and not exposed via Objective-C or Kotlin/Native cinterop, so the integration uses a tiny bridge:

sample-ios/AppleFoundationBridgeImpl.swift   ── implements ──► Kotlin AppleFoundationBridge protocol
                                                                       │
halogen-provider-apple-foundation                                      │
  AppleFoundationProvider(bridge) : HalogenLlmProvider  ◄──────────────┘
    suspendCancellableCoroutine wraps the callback-based bridge

Swift's LanguageModelSession.respond(to:) is async throws; wrapping with Task { ... completion(...) } produces an ObjC-block-friendly API that exports cleanly to Kotlin, which then re-presents it as a suspend function.

Availability mapping (Swift → Kotlin):

• .available → HalogenLlmAvailability.READY
• .unavailable(.modelNotReady) → INITIALIZING
• .unavailable(.deviceNotEligible | .appleIntelligenceNotEnabled) → UNAVAILABLE
• iOS < 26 (no FoundationModels) → UNAVAILABLE

The Swift bridge also exposes makeIfAvailable() which returns nil unless the model is actually .available at construction time, letting ContentView.swift cleanly hand a nil bridge to Kotlin and skip the provider entirely.

Files

New

• halogen-provider-apple-foundation/build.gradle.kts — iOS-only KMP module (applies kotlin.multiplatform directly; the halogen.kmp-library convention is intentionally skipped because it would pull in Android/JVM/wasmJs).
• halogen-provider-apple-foundation/src/iosMain/kotlin/halogen/provider/apple/foundation/AppleFoundationBridge.kt — public Kotlin protocol that Swift implements.
• halogen-provider-apple-foundation/src/iosMain/kotlin/halogen/provider/apple/foundation/AppleFoundationProvider.kt — HalogenLlmProvider impl adapting callbacks → suspend.
• halogen-provider-apple-foundation/src/iosTest/kotlin/halogen/provider/apple/foundation/AppleFoundationProviderTest.kt — fake bridge covering success, NSError mapping, empty response, and each availability state.
• sample-ios/iosApp/AppleFoundationBridgeImpl.swift — Swift bridge that actually calls FoundationModels.

Modified

• settings.gradle.kts — register the new module.
• sample-shared/build.gradle.kts — depend on :halogen-provider-apple-foundation in iosMain.
• sample-shared/.../HalogenDemoState.kt — create(...) now accepts an optional preferredProvider so iOS can supply Apple FM; previous fallback to OpenAI / Demo is preserved.
• sample-shared/.../MainViewController.kt — accepts appleFoundationBridge: AppleFoundationBridge? = null and uses it when non-null.
• sample-ios/iosApp/ContentView.swift — constructs the bridge via AppleFoundationBridgeImpl.makeIfAvailable().
• sample-ios/project.yml — adds -weak_framework FoundationModels so the app still launches on iOS 16-25.

Test plan

• ./gradlew :halogen-provider-apple-foundation:build — compiles iOS targets and runs the unit tests with a fake bridge (no simulator required).
• ./gradlew :sample-shared:embedAndSignAppleFrameworkForXcode — confirm the AppleFoundationBridge protocol shows up in the generated HalogenSample umbrella header.
• Open sample-ios in Xcode; build on an iOS 26 simulator with Apple Intelligence enabled — confirm providerName reads "Apple Foundation Models" and a prompt like "warm coffee shop" produces a Material 3 theme.
• iOS 26 simulator with Apple Intelligence disabled — confirm graceful fallback to OpenAI / Demo with no crash.
• iOS 16 simulator — confirm the app launches (weak-link works) and falls back to OpenAI / Demo.

https://claude.ai/code/session_01AvYU38211BVwAyWNXFEvvW

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