Add native buffered file subscriber

docs/file.md

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+# File subscriber
+
+`brickfrog/moontrace/file` is a native-only buffered file subscriber. The
+subscriber returned by `FileSubscriber::subscriber()` is synchronous: it formats
+the event as one JSON line and attempts a non-blocking enqueue. When the queue
+is full or shut down, the event is dropped and counted.
+
+The application owns the async worker. Start `run()` inside the app's async
+context, then call `flush()` at durability boundaries and `shutdown()` before
+leaving the task group.
+
+```moonbit
+let files = @file.file_subscriber(
+  "logs/moontrace.jsonl",
+  max_size_bytes=10 * 1024 * 1024,
+  max_files=5,
+  gzip=true,
+  max_batch=128,
+  queue_capacity=1024,
+)
+
+@moontrace.set_subscriber(files.subscriber())
+
+@async.with_task_group(group => {
+  group.spawn_bg(() => files.run())
+
+  @moontrace.info("service started")
+  files.flush()
+
+  files.shutdown()
+})
+```
+
+Rotation is size-based. When the active file crosses `max_size_bytes`, it is
+renamed to `path.1`, older rotated files are shifted up, and files beyond
+`max_files` are removed. With `gzip=true`, rotated files are written as
+`path.N.gz` and the temporary uncompressed rotation is removed.
+
+The file subscriber uses `moonbitlang/async/fs` and is only supported on the
+native target. Constructing it on other targets aborts immediately with a clear
+unsupported-target error instead of returning a subscriber that drops events.