📝 document file sink counters and resets

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

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+---
+name: file-sink-write-failures
+group: api
+category: sink
+update-time: 20260613
+description: Read the number of write failures recorded by a FileSink.
+key-word:
+    - file
+    - sink
+    - write
+    - public
+---
+
+## File-sink-write-failures
+
+Read the number of write failures recorded by a `FileSink`. This helper is useful for diagnosing direct runtime write-path problems.
+
+### Interface
+
+```moonbit
+pub fn FileSink::write_failures(self : FileSink) -> Int {
+```
+
+#### input
+
+- `self : FileSink` - File sink whose write-failure counter should be inspected.
+
+#### output
+
+- `Int` - Number of recorded write failures.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- The sink reports its recorded write-failure count directly.
+- The counter is cumulative until reset.
+- This helper is observation-only and does not mutate sink state.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Runtime Write-path Diagnostics
+
+When support output should show whether direct writes have been failing:
+```moonbit
+let count = sink.write_failures()
+```
+
+In this example, callers get a focused signal for direct file write-path health.
+
+#### When Compare Sink Health After Recovery
+
+When recovery logic should verify whether new failures still occur:
+```moonbit
+ignore(sink.write_failures())
+```
+
+In this example, the metric helps measure whether runtime writes improved after intervention.
+
+### Error Case
+
+e.g.:
+- If callers need a fuller view of file health, `state()` may be a better API.
+
+- This counter does not itself distinguish open-path issues from write-path issues.
+
+### Notes
+
+1. Use this helper for focused write-failure visibility.
+
+2. Pair it with `flush_failures()` when diagnosing direct output-path instability.