📝 document wrapper sink types

docs/api/queued-sink-type.md

@@ -0,0 +1,79 @@
+---
+name: queued-sink-type
+group: api
+category: sink
+update-time: 20260613
+description: Public queued sink type used for explicit synchronous queueing over another sink.
+key-word:
+    - sink
+    - queue
+    - type
+    - public
+---
+
+## Queued-sink-type
+
+`QueuedSink[S]` is the public queue wrapper sink type used for explicit synchronous queueing over another sink. It is the concrete sink type returned by `queued_sink(...)` and preserves the wrapped sink type in its type parameter.
+
+### Interface
+
+```moonbit
+pub struct QueuedSink[S] {
+  sink : S
+  queue : @queue.Queue[Record]
+  max_pending : Int
+  overflow : QueueOverflowPolicy
+  dropped_count : Ref[Int]
+}
+```
+
+#### output
+
+- `QueuedSink[S]` - Public synchronous sink type that queues records before they are drained to the wrapped sink.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- This is a public root struct, not a type alias.
+- The current fields are `sink : S`, `queue : @queue.Queue[Record]`, `max_pending : Int`, `overflow : QueueOverflowPolicy`, and `dropped_count : Ref[Int]`.
+- `queued_sink(...)` constructs this type directly from a wrapped sink and queue policy.
+- The sink preserves the wrapped sink type `S`, which is useful when typed composition still matters after queueing is introduced.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need A Typed Queued Sink Value
+
+When code should keep the concrete queue wrapper sink type visible:
+```moonbit
+let sink : QueuedSink[ConsoleSink] = queued_sink(console_sink(), max_pending=4)
+```
+
+In this example, the sink value stays explicit and preserves the wrapped console sink type.
+
+#### When Need A Typed Logger With Explicit Queueing
+
+When logging should preserve the queue wrapper sink type in the logger:
+```moonbit
+let logger : Logger[QueuedSink[ConsoleSink]] = Logger::new(
+  queued_sink(console_sink(), max_pending=2, overflow=QueueOverflowPolicy::DropOldest),
+  target="queue",
+)
+```
+
+In this example, the concrete queued sink remains part of the logger type.
+
+### Error Case
+
+e.g.:
+- `QueuedSink[S]` itself does not have a runtime failure mode.
+
+- Runtime behavior still depends on the wrapped sink `S`, and records may be dropped when the queue reaches the configured bound.
+
+### Notes
+
+1. Use `queued_sink(...)` when you need a value of this type.
+
+2. Use `BufferedSink[S]` when simple threshold buffering is enough and overflow policy is unnecessary.