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/impl/build.gradle.kts

@@ -0,0 +1,29 @@
+import extension.setupDependencyInjection
+import extension.testCommonDependencies
+
+/*
+ * 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.impl"
+}
+
+setupDependencyInjection()
+
+dependencies {
+    api(projects.features.callnative.api)
+
+    implementation(projects.features.call.api)
+    implementation(projects.libraries.architecture)
+    implementation(projects.libraries.di)
+    implementation(projects.libraries.matrix.api)
+
+    testCommonDependencies(libs)
+}