📝 Add async state and config export API docs

docs/api/async-logger-build-config-to-json.md

@@ -0,0 +1,104 @@
+---
+name: async-logger-build-config-to-json
+group: api
+category: async
+update-time: 20260512
+description: Convert AsyncLoggerBuildConfig into a JSON value for exporting complete async logger build settings.
+key-word:
+    - async
+    - build
+    - config
+    - public
+---
+
+## Async-logger-build-config-to-json
+
+Convert `AsyncLoggerBuildConfig` into a `JsonValue`. This helper exports both the base synchronous logger config and the async runtime config as one structured payload.
+
+### Interface
+
+```moonbit
+pub fn async_logger_build_config_to_json(
+  config : AsyncLoggerBuildConfig,
+) -> @json_parser.JsonValue {}
+```
+
+#### input
+
+- `config : AsyncLoggerBuildConfig` - Complete build config used by async logger builders.
+
+#### output
+
+- `JsonValue` - Structured JSON representation of the full async build config.
+
+---
+
+e.g.:
+```moonbit
+pub fn async_logger_build_config_to_json(config : AsyncLoggerBuildConfig) -> @json_parser.JsonValue {}
+```
+
+#### input
+
+- `config : AsyncLoggerBuildConfig` - Combined logger and async config.
+
+#### output
+
+- `JsonValue` - JSON-exportable build payload.
+
+---
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- The output always includes `logger` and `async_config`.
+- Logger export is delegated to `@bitlogger.logger_config_to_json(...)`.
+- Async export is delegated to `async_logger_config_to_json(...)`.
+- This helper is useful when generated setup should preserve both sink/logger behavior and async runtime behavior together.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Structured Export Of Full Async Setup
+
+When a tool or test needs one object describing the whole async logger build:
+```moonbit
+let payload = async_logger_build_config_to_json(
+  AsyncLoggerBuildConfig::new(
+    logger=@bitlogger.LoggerConfig::new(target="svc"),
+    async_config=AsyncLoggerConfig::new(max_pending=64),
+  ),
+)
+```
+
+In this example, both layers of configuration are exported together.
+
+#### When Need Roundtrip-friendly Build Config Data
+
+When generated build config should later be parsed again:
+```moonbit
+let value = async_logger_build_config_to_json(AsyncLoggerBuildConfig::new())
+```
+
+In this example, the resulting JSON matches the supported async build config shape.
+
+### Error Case
+
+e.g.:
+- If callers only need the async runtime section, this API is broader than necessary and `async_logger_config_to_json(...)` should be used instead.
+
+- If callers want direct text output, they should use `stringify_async_logger_build_config(...)` instead.
+
+### Notes
+
+Notes are here.
+
+1. This helper exports complete build settings, not runtime state.
+
+2. It is useful for generated configs, test fixtures, and setup introspection.
+
+3. The output combines `LoggerConfig` and `AsyncLoggerConfig` without flattening them.
+
+4. This API complements `parse_async_logger_build_config_text(...)` in roundtrip workflows.