Add cross-target async compatibility runtime

bitlogger_async/BitLoggerAsync_test.mbt

@@ -1,4 +1,5 @@
 async test "shutdown drains pending records" {
+  inspect(async_runtime_mode_label(async_runtime_mode()) == "native_worker" || async_runtime_mode_label(async_runtime_mode()) == "compatibility", content="true")
   let written : Ref[Array[String]] = Ref::new([])
   let flushes : Ref[Int] = Ref::new(0)
   let logger = async_logger(
@@ -148,3 +149,59 @@ test "async build config stringify roundtrips nested logger and async fields" {
     AsyncFlushPolicy::Shutdown => "Shutdown"
   }, content="Shutdown")
 }
+
+test "async runtime capability helpers stay consistent" {
+  let mode = async_runtime_mode()
+  let state = async_runtime_state()
+  let worker_supported = match mode {
+    AsyncRuntimeMode::NativeWorker => true
+    AsyncRuntimeMode::Compatibility => false
+  }
+  inspect(
+    async_runtime_mode_label(mode) == "native_worker" || async_runtime_mode_label(mode) == "compatibility",
+    content="true",
+  )
+  inspect(async_runtime_supports_background_worker() == worker_supported, content="true")
+  inspect(async_runtime_mode_label(state.mode) == async_runtime_mode_label(mode), content="true")
+  inspect(state.background_worker == worker_supported, content="true")
+  inspect(
+    stringify_async_runtime_state(state),
+    content=if worker_supported {
+      "{\"mode\":\"native_worker\",\"background_worker\":true}"
+    } else {
+      "{\"mode\":\"compatibility\",\"background_worker\":false}"
+    },
+  )
+}
+
+async test "run drains queued records in compatibility backends too" {
+  let written : Ref[Array[String]] = Ref::new([])
+  let logger = async_logger(
+    @bitlogger.callback_sink(fn(rec) {
+      written.val.push(rec.message)
+    }),
+    config=AsyncLoggerConfig::new(
+      max_pending=4,
+      overflow=AsyncOverflowPolicy::DropNewest,
+      max_batch=2,
+      linger_ms=5,
+      flush=AsyncFlushPolicy::Never,
+    ),
+    min_level=@bitlogger.Level::Info,
+    target="async.compat",
+  )
+
+  @async.with_task_group(group => {
+    logger.info("one")
+    logger.info("two")
+    inspect(logger.pending_count(), content="2")
+    group.spawn_bg(() => logger.run())
+    logger.shutdown()
+  })
+
+  inspect(logger.is_closed(), content="true")
+  inspect(logger.pending_count(), content="0")
+  inspect(written.val.length(), content="2")
+  inspect(written.val[0], content="one")
+  inspect(written.val[1], content="two")
+}