📝 Add async state and config export API docs

docs/api/async-runtime-state-to-json.md

@@ -0,0 +1,97 @@
+---
+name: async-runtime-state-to-json
+group: api
+category: async
+update-time: 20260512
+description: Convert AsyncRuntimeState into a JSON value for runtime capability and mode diagnostics.
+key-word:
+    - async
+    - state
+    - json
+    - public
+---
+
+## Async-runtime-state-to-json
+
+Convert `AsyncRuntimeState` into a `JsonValue`. This helper exports the async runtime mode and background worker capability in a structured form.
+
+### Interface
+
+```moonbit
+pub fn async_runtime_state_to_json(state : AsyncRuntimeState) -> @json_parser.JsonValue {}
+```
+
+#### input
+
+- `state : AsyncRuntimeState` - Runtime capability snapshot, usually produced by `async_runtime_state()`.
+
+#### output
+
+- `JsonValue` - Structured JSON representation of the runtime state.
+
+---
+
+e.g.:
+```moonbit
+pub fn async_runtime_state_to_json(state : AsyncRuntimeState) -> @json_parser.JsonValue {}
+```
+
+#### input
+
+- `state : AsyncRuntimeState` - Runtime snapshot to export.
+
+#### output
+
+- `JsonValue` - JSON-exportable state object.
+
+---
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- The output includes `mode` and `background_worker`.
+- `mode` is serialized through `async_runtime_mode_label(...)`.
+- This helper focuses on runtime capabilities rather than queue counters or logger lifecycle flags.
+- The exported JSON is suitable for diagnostics endpoints and startup environment checks.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Structured Runtime Capability Checks
+
+When startup diagnostics should include async runtime capability data:
+```moonbit
+let runtime_json = async_runtime_state_to_json(async_runtime_state())
+```
+
+In this example, the runtime snapshot becomes a reusable JSON value.
+
+#### When Need To Embed Runtime State In A Larger Payload
+
+When async support should be one field in a bigger diagnostics object:
+```moonbit
+let payload = async_runtime_state_to_json(state)
+```
+
+In this example, callers can reuse the exported object directly.
+
+### Error Case
+
+e.g.:
+- If callers expect logger queue counters or failure status, this API is too narrow and `async_logger_state_to_json(...)` should be used instead.
+
+- If the runtime is in compatibility mode, the helper still serializes normally using the matching mode label.
+
+### Notes
+
+Notes are here.
+
+1. This helper exports runtime capability state, not logger workload state.
+
+2. Use it with `async_runtime_state()` for a fresh snapshot.
+
+3. Prefer `stringify_async_runtime_state(...)` when immediate text output is enough.
+
+4. The output is useful for environment diagnostics across targets.