📝 Add async state and config export API docs

docs/api/stringify-async-logger-state.md

@@ -0,0 +1,102 @@
+---
+name: stringify-async-logger-state
+group: api
+category: async
+update-time: 20260512
+description: Serialize AsyncLoggerState into compact or pretty JSON text for logs and diagnostics.
+key-word:
+    - async
+    - stringify
+    - state
+    - public
+---
+
+## Stringify-async-logger-state
+
+Serialize `AsyncLoggerState` into JSON text. This is the highest-level diagnostic export helper for async logger snapshots when callers want immediate text output.
+
+### Interface
+
+```moonbit
+pub fn stringify_async_logger_state(
+  state : AsyncLoggerState,
+  pretty~ : Bool = false,
+) -> String {}
+```
+
+#### input
+
+- `state : AsyncLoggerState` - Snapshot produced by `AsyncLogger::state()`.
+- `pretty : Bool` - Whether JSON should be pretty-printed.
+
+#### output
+
+- `String` - JSON text for the async logger snapshot.
+
+---
+
+e.g.:
+```moonbit
+pub fn stringify_async_logger_state(state : AsyncLoggerState, pretty~ : Bool = false) -> String {}
+```
+
+#### input
+
+- `state : AsyncLoggerState` - Snapshot to serialize.
+- `pretty : Bool` - Pretty-print flag.
+
+#### output
+
+- `String` - Serialized JSON text.
+
+---
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- `pretty=false` returns compact JSON.
+- `pretty=true` returns indented JSON suitable for human diagnostics.
+- This helper is built on top of `async_logger_state_to_json(...)`.
+- String output is convenient for logs, CLI output, and startup diagnostics.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Human-readable Diagnostics
+
+When logger state should be printed for humans:
+```moonbit
+println(stringify_async_logger_state(logger.state(), pretty=true))
+```
+
+In this example, the pretty output is suitable for startup or incident diagnostics.
+
+#### When Need Compact Structured Logs
+
+When snapshot text should stay small:
+```moonbit
+println(stringify_async_logger_state(logger.state()))
+```
+
+In this example, compact JSON is emitted without extra formatting logic.
+
+### Error Case
+
+e.g.:
+- If the snapshot contains an empty `last_error`, it is still included as an empty string in the serialized output.
+
+- If callers need a JSON value instead of text, they should use `async_logger_state_to_json(...)` instead.
+
+### Notes
+
+Notes are here.
+
+1. This API is the most convenient direct output path for async logger snapshots.
+
+2. Use `pretty=true` for humans and the default compact mode for machine-oriented logs.
+
+3. Pair with `AsyncLogger::state()` to capture a consistent point-in-time snapshot.
+
+4. This helper is ideal for startup, shutdown, and failure reporting.