⬆️ upgrade async and replace native file FFI

升级 async 依赖，迁移到 MoonBit 原生 async/fs 文件接口，移除项目自有 C FFI，并固定文本文件为 LF。CI 全部通过。

docs/api/file-sink-async.md

@@ -0,0 +1,65 @@
+---
+name: file-sink-async
+group: api
+category: sink
+update-time: 20260825
+description: Create a file sink from an async context using the native async filesystem interface.
+key-word:
+    - file
+    - sink
+    - async
+    - public
+---
+
+## File-sink-async
+
+Create a `FileSink` from an async context. The constructor opens the file with the native `moonbitlang/async/fs` interface and keeps the same rotation, policy, and failure-counter surface as `file_sink(...)`.
+
+### Interface
+
+```moonbit
+pub async fn file_sink_async(
+  path : String,
+  append~ : Bool = true,
+  auto_flush~ : Bool = true,
+  rotation~ : FileRotation? = None,
+  formatter~ : RecordFormatter = fn(rec) { format_text(rec) },
+) -> FileSink {}
+```
+
+#### input
+
+- `path : String` - Destination file path.
+- `append : Bool` - Whether writes append rather than truncate on the first open.
+- `auto_flush : Bool` - Whether each async write requests an async file sync.
+- `rotation : FileRotation?` - Optional size-based rotation policy.
+- `formatter : RecordFormatter` - Formatter used to render each record.
+
+#### output
+
+- `FileSink` - A file sink ready for use by an async logger or direct async sink writes.
+
+### Explanation
+
+- Use this constructor when the caller is already running inside an async event loop.
+- File writes, flushes, file-size checks, renames, removals, and rotation reopening use the async filesystem path.
+- The returned sink implements `Sink::write_async(...)`; the synchronous `write(...)` surface remains available for synchronous callers.
+- On non-native targets, the sink follows the existing unavailable-backend behavior.
+
+### How to Use
+
+```moonbit
+let sink = file_sink_async("app.log", auto_flush=false)
+let logger = async_logger(sink)
+```
+
+### Error Case
+
+- If the initial async open fails, the returned sink is unavailable and increments `open_failures()`.
+- Later async writes record write and rotation failures through the existing sink counters.
+
+### Notes
+
+1. Use `file_sink(...)` for synchronous construction outside an async event loop.
+
+2. Pair this API with `native_files_supported()` when code must also compile for non-native targets.

docs/api/file-sink.md

@@ -98,3 +98,5 @@ e.g.:
 3. Non-native targets can still compile code referencing this API, but callers should treat actual file availability and successful writes as target-sensitive runtime behavior.
 
 4. See [target-verification.md](./target-verification.md) for the current verification boundary between design intent and locally re-checked targets.
+
+5. Use [`file_sink_async(...)`](./file-sink-async.md) when constructing a file sink from an async event loop.

docs/api/index.md

@@ -153,6 +153,7 @@ BitLogger API navigation.
 - [patch-sink.md](./patch-sink.md)
 - [patch-sink-type.md](./patch-sink-type.md)
 - [file-sink.md](./file-sink.md)
+- [file-sink-async.md](./file-sink-async.md)
 - [file-sink-available.md](./file-sink-available.md)
 - [file-sink-flush.md](./file-sink-flush.md)
 - [file-sink-close.md](./file-sink-close.md)