📝 document runtime sink failure counters

docs/api/runtime-sink-file-write-failures.md

@@ -0,0 +1,74 @@
+---
+name: runtime-sink-file-write-failures
+group: api
+category: runtime
+update-time: 20260613
+description: Read the number of write failures recorded by a file-backed RuntimeSink.
+key-word:
+    - runtime
+    - sink
+    - file
+    - public
+---
+
+## Runtime-sink-file-write-failures
+
+Read the number of write failures recorded by a file-backed `RuntimeSink`. This helper is useful for direct runtime diagnostics around the file write path.
+
+### Interface
+
+```moonbit
+pub fn RuntimeSink::file_write_failures(self : RuntimeSink) -> Int {
+```
+
+#### input
+
+- `self : RuntimeSink` - Runtime sink whose write-failure counter should be inspected.
+
+#### output
+
+- `Int` - Number of recorded write failures.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- Plain `File` runtime variants report the wrapped `FileSink` write-failure count.
+- `QueuedFile` runtime variants forward the metric from the wrapped inner `FileSink`.
+- Non-file runtime variants return `0`.
+- The counter is cumulative until `file_reset_failure_counters()` clears it.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Runtime Write-path Diagnostics
+
+When support output should show whether file writes have been failing:
+```moonbit
+let count = sink.file_write_failures()
+```
+
+In this example, callers get a focused signal for direct runtime file write-path health.
+
+#### When Compare Sink Health After Recovery
+
+When recovery logic should verify whether new failures still occur:
+```moonbit
+ignore(sink.file_write_failures())
+```
+
+In this example, the metric helps measure whether runtime writes improved after intervention.
+
+### Error Case
+
+e.g.:
+- If the runtime sink is not file-backed, the method returns `0`.
+
+- If callers need a fuller view of file health, `file_state()` or `file_runtime_state()` may be better APIs.
+
+### Notes
+
+1. Use this helper for focused direct runtime write-failure visibility.
+
+2. Pair it with `file_flush_failures()` when diagnosing output-path instability.