Route calls through an entry point the native stack can implement

Adds the seam a native MatrixRTC call stack plugs into, with nothing behind it
yet, so that the stack itself lands as a self-contained change.

`FeatureFlags.NativeCall` is off by default and unfinished.
`features/callnative/api` declares `NativeCallEntryPoint`, and
`features/callnative/impl` provides a default that logs and does nothing:
turning the flag on today gets no call and a line in the log. That is the
honest outcome, where falling back to the WebView would make the flag look
broken rather than unimplemented.

`DefaultElementCallEntryPoint` forks on the flag. The read is asynchronous
because the flag is storage-backed, and caching it in an eagerly started
StateFlow would race a call placed just after app start onto the default
value.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

features/callnative/api/build.gradle.kts

@@ -0,0 +1,21 @@
+/*
+ * Copyright (c) 2026 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+plugins {
+    id("io.element.android-library")
+}
+
+android {
+    namespace = "io.element.android.features.callnative.api"
+}
+
+dependencies {
+    // api, not implementation: CallData is part of NativeCallEntryPoint's signature.
+    api(projects.features.call.api)
+    implementation(projects.libraries.architecture)
+    implementation(projects.libraries.matrix.api)
+}

features/callnative/api/src/main/kotlin/io/element/android/features/callnative/api/NativeCallEntryPoint.kt

@@ -0,0 +1,24 @@
+/*
+ * Copyright (c) 2026 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+package io.element.android.features.callnative.api
+
+import io.element.android.features.call.api.CallData
+
+/**
+ * Starts a call on the native MatrixRTC stack instead of the Element Call WebView.
+ *
+ * Experimental: gated behind `FeatureFlags.NativeCall` and reached only through
+ * [io.element.android.features.call.api.ElementCallEntryPoint], so nothing else in the app has to know
+ * it exists.
+ *
+ * There is nothing behind this seam yet - the default implementation does nothing and says so. The
+ * native stack that implements it is a separate library, added in a follow-up.
+ */
+interface NativeCallEntryPoint {
+    fun startCall(callData: CallData)
+}