📝 document async library facade composition apis

docs/api/library-async-logger-with-target.md

@@ -0,0 +1,84 @@
+---
+name: library-async-logger-with-target
+group: api
+category: facade
+update-time: 20260613
+description: Replace the default target carried by a LibraryAsyncLogger facade.
+key-word:
+    - async
+    - library
+    - facade
+    - public
+---
+
+## Library-async-logger-with-target
+
+Replace the default target on a `LibraryAsyncLogger[S]`. This is the library-facing counterpart to `AsyncLogger::with_target(...)` when package code wants to retarget an async logger without exposing the full async logger API.
+
+### Interface
+
+```moonbit
+pub fn[S] LibraryAsyncLogger::with_target(
+  self : LibraryAsyncLogger[S],
+  target : String,
+) -> LibraryAsyncLogger[S] {
+```
+
+#### input
+
+- `self : LibraryAsyncLogger[S]` - Base facade whose default target should be replaced.
+- `target : String` - New default target namespace.
+
+#### output
+
+- `LibraryAsyncLogger[S]` - New library-facing async facade carrying the updated target.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- This API delegates to the wrapped async logger's `with_target(...)` behavior and then re-wraps the result.
+- The returned value keeps the same queue state, sink type, and async config.
+- This replaces the default target instead of composing it.
+- The original facade value is not mutated.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need A New Fixed Async Namespace Inside Library Code
+
+When one facade should emit under a different async target namespace:
+```moonbit
+let logger = LibraryAsyncLogger::new(@bitlogger.console_sink())
+  .with_target("sdk.http")
+```
+
+In this example, later async writes inherit `sdk.http` unless a call overrides the target directly.
+
+#### When Reuse One Async Setup Across Library Subsystems
+
+When multiple library-facing async loggers should share one queue policy:
+```moonbit
+let base = LibraryAsyncLogger::new(
+  @bitlogger.console_sink(),
+  config=AsyncLoggerConfig::new(max_pending=64),
+)
+let cache = base.with_target("cache")
+let io = base.with_target("io")
+```
+
+In this example, one base facade becomes several target-specific async facades.
+
+### Error Case
+
+e.g.:
+- If `target` is empty, the returned logger remains valid and later records simply use an empty default target.
+
+- If callers need hierarchical target composition rather than replacement, `child(...)` is the better API.
+
+### Notes
+
+1. Use this API for replacement, not target-path composition.
+
+2. It keeps the narrower `LibraryAsyncLogger` boundary intact.