📝 document library facade conversion apis

docs/api/logger-to-library-logger.md

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+---
+name: logger-to-library-logger
+group: api
+category: facade
+update-time: 20260613
+description: Convert a full sync logger into the narrower library-facing sync facade.
+key-word:
+    - logger
+    - library
+    - facade
+    - public
+---
+
+## Logger-to-library-logger
+
+Convert `Logger[S]` into `LibraryLogger[S]`. This keeps the same sink and logging behavior while projecting the value onto the smaller library-facing sync surface.
+
+### Interface
+
+```moonbit
+pub fn[S] Logger::to_library_logger(self : Logger[S]) -> LibraryLogger[S] {}
+```
+
+#### input
+
+- `self : Logger[S]` - Full sync logger to project into the library facade.
+
+#### output
+
+- `LibraryLogger[S]` - Narrower library-facing wrapper over the same logger state.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- This conversion does not rebuild the sink or change the logger configuration.
+- Target, min level, timestamp behavior, and sink wiring are preserved.
+- The returned facade keeps library-oriented write APIs such as `info(...)`, `warn(...)`, and `error(...)`.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need To Return A Narrower Library Type
+
+When internal setup uses the full logger API but the exposed result should stay smaller:
+```moonbit
+let logger = Logger::new(console_sink(), target="lib")
+let public_logger = logger.to_library_logger()
+```
+
+In this example, `public_logger` keeps the same logging behavior but exposes the library facade.
+
+### Error Case
+
+e.g.:
+- If callers later need APIs outside the library facade, they must unwrap with `to_logger()`.
+
+- The conversion does not remove existing target or field bindings from the original logger.
+
+### Notes
+
+1. Use this when library boundaries should avoid exposing the full sync logger surface.
+
+2. This is a projection API, not a copy or rebuild step.