📝 document runtime sink auto flush methods

docs/api/index.md

@@ -361,6 +361,8 @@ BitLogger API navigation.
 - [runtime-sink-close.md](./runtime-sink-close.md)
 - [runtime-sink-pending-count.md](./runtime-sink-pending-count.md)
 - [runtime-sink-dropped-count.md](./runtime-sink-dropped-count.md)
+- [runtime-sink-file-auto-flush.md](./runtime-sink-file-auto-flush.md)
+- [runtime-sink-file-set-auto-flush.md](./runtime-sink-file-set-auto-flush.md)
 - [configured-logger.md](./configured-logger.md)
 - [configured-logger-flush.md](./configured-logger-flush.md)
 - [configured-logger-drain.md](./configured-logger-drain.md)

docs/api/runtime-sink-file-auto-flush.md

@@ -0,0 +1,75 @@
+---
+name: runtime-sink-file-auto-flush
+group: api
+category: runtime
+update-time: 20260613
+description: Read whether the file-backed RuntimeSink currently has auto-flush enabled.
+key-word:
+    - runtime
+    - sink
+    - file
+    - public
+---
+
+## Runtime-sink-file-auto-flush
+
+Read whether auto-flush is currently enabled on a file-backed `RuntimeSink`. This helper exposes one important direct runtime durability policy flag without going through `ConfiguredLogger`.
+
+### Interface
+
+```moonbit
+pub fn RuntimeSink::file_auto_flush(self : RuntimeSink) -> Bool {
+```
+
+#### input
+
+- `self : RuntimeSink` - Runtime sink whose file auto-flush policy should be inspected.
+
+#### output
+
+- `Bool` - Whether auto-flush is currently enabled.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- Plain `File` runtime variants report the current auto-flush policy from the wrapped `FileSink`.
+- `QueuedFile` runtime variants forward the policy from the wrapped inner `FileSink`.
+- Non-file runtime variants return `false`.
+- This helper exposes policy state only and does not force any flush action.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Direct Runtime Durability Visibility
+
+When code owns a `RuntimeSink` directly and should expose whether each file write auto-flushes:
+```moonbit
+let enabled = sink.file_auto_flush()
+```
+
+In this example, runtime file durability policy is surfaced without going through a logger wrapper.
+
+#### When Validate Direct Policy Updates
+
+When code should observe auto-flush after a direct runtime setter call:
+```moonbit
+ignore(sink.file_set_auto_flush(true))
+ignore(sink.file_auto_flush())
+```
+
+In this example, callers verify the updated policy on the runtime sink itself.
+
+### Error Case
+
+e.g.:
+- If the runtime sink is not file-backed, the method returns `false`.
+
+- If callers need to actually flush file output, `file_flush()` is the operational API.
+
+### Notes
+
+1. Use this helper to inspect direct runtime durability policy on `RuntimeSink` values.
+
+2. It complements `file_set_auto_flush(...)` rather than replacing real flush actions.

docs/api/runtime-sink-file-set-auto-flush.md

@@ -0,0 +1,75 @@
+---
+name: runtime-sink-file-set-auto-flush
+group: api
+category: runtime
+update-time: 20260613
+description: Update the auto-flush policy used by a file-backed RuntimeSink.
+key-word:
+    - runtime
+    - sink
+    - file
+    - public
+---
+
+## Runtime-sink-file-set-auto-flush
+
+Update the auto-flush policy used by a file-backed `RuntimeSink`. This helper changes direct runtime durability behavior without rebuilding or rewrapping the sink.
+
+### Interface
+
+```moonbit
+pub fn RuntimeSink::file_set_auto_flush(self : RuntimeSink, enabled : Bool) -> Bool {
+```
+
+#### input
+
+- `self : RuntimeSink` - Runtime sink whose file auto-flush policy should change.
+- `enabled : Bool` - New auto-flush setting.
+
+#### output
+
+- `Bool` - Whether the policy update was applied.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- Plain `File` runtime variants update the wrapped `FileSink` auto-flush policy and return `true`.
+- `QueuedFile` runtime variants forward the update to the wrapped inner `FileSink` and return `true`.
+- Non-file runtime variants return `false`.
+- This helper changes policy only; it does not itself flush pending data.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Direct Runtime Durability Tuning
+
+When a file-backed runtime sink should start flushing each write automatically:
+```moonbit
+ignore(sink.file_set_auto_flush(true))
+```
+
+In this example, runtime durability policy is updated without rebuilding the sink.
+
+#### When Need To Relax Flush Pressure On A Runtime Sink
+
+When file writes should stop auto-flushing for throughput reasons:
+```moonbit
+let ok = sink.file_set_auto_flush(false)
+```
+
+In this example, the call site updates policy explicitly and can inspect the result.
+
+### Error Case
+
+e.g.:
+- If the runtime sink is not file-backed, the method returns `false`.
+
+- If callers need an immediate flush action, `file_flush()` is the operational API.
+
+### Notes
+
+1. Use this helper for direct runtime durability tuning on `RuntimeSink` values.
+
+2. Pair it with `file_auto_flush()` to verify the active policy.