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>

libraries/featureflag/api/src/main/kotlin/io/element/android/libraries/featureflag/api/FeatureFlags.kt

@@ -162,4 +162,11 @@ enum class FeatureFlags(
         isFinished = false,
         isInLabs = false,
     ),
+    NativeCall(
+        key = "feature.native_call",
+        title = "Native calls (experimental)",
+        description = "Place calls with the native MatrixRTC stack instead of the Element Call WebView.",
+        defaultValue = { false },
+        isFinished = false,
+    ),
 }