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/src/main/kotlin/io/element/android/features/callnative/impl/DefaultNativeCallEntryPoint.kt

@@ -0,0 +1,29 @@
+/*
+ * 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.impl
+
+import dev.zacsweers.metro.AppScope
+import dev.zacsweers.metro.ContributesBinding
+import io.element.android.features.call.api.CallData
+import io.element.android.features.callnative.api.NativeCallEntryPoint
+import timber.log.Timber
+
+/**
+ * What answers [NativeCallEntryPoint] until the native MatrixRTC stack is wired in: nothing.
+ *
+ * The seam exists so that `DefaultElementCallEntryPoint` can fork on `FeatureFlags.NativeCall` today,
+ * and so that adding the stack later is a change to this module alone. A user who turns the flag on
+ * before then gets no call and a line in the log, which is the honest outcome - falling back to the
+ * WebView would make the flag look broken rather than unimplemented.
+ */
+@ContributesBinding(AppScope::class)
+class DefaultNativeCallEntryPoint : NativeCallEntryPoint {
+    override fun startCall(callData: CallData) {
+        Timber.w("Native call requested for ${callData.roomId} but no native call stack is present in this build")
+    }
+}