📝 Add configured logger file state API docs

docs/api/configured-logger-file-write-failures.md

@@ -0,0 +1,74 @@
+---
+name: configured-logger-file-write-failures
+group: api
+category: runtime
+update-time: 20260512
+description: Read the number of write failures recorded by the configured runtime file sink.
+key-word:
+    - logger
+    - runtime
+    - file
+    - public
+---
+
+## Configured-logger-file-write-failures
+
+Read the number of write failures recorded by a `ConfiguredLogger` file sink. This helper is useful for diagnosing runtime write-path problems.
+
+### Interface
+
+```moonbit
+pub fn ConfiguredLogger::file_write_failures(self : ConfiguredLogger) -> Int {}
+```
+
+#### input
+
+- `self : ConfiguredLogger` - Config-driven runtime logger whose write-failure counter should be inspected.
+
+#### output
+
+- `Int` - Number of recorded write failures.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- File-backed sinks report their recorded write-failure count.
+- Queued file sinks forward the metric from the wrapped file sink.
+- Non-file sinks return `0`.
+- The counter is cumulative until reset.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Runtime Write-path Diagnostics
+
+When support output should show whether writes have been failing:
+```moonbit
+let count = logger.file_write_failures()
+```
+
+In this example, callers get a focused signal for file write-path health.
+
+#### When Compare Sink Health After Recovery
+
+When recovery logic should verify whether new failures still occur:
+```moonbit
+ignore(logger.file_write_failures())
+```
+
+In this example, the metric helps measure whether runtime writes improved after intervention.
+
+### Error Case
+
+e.g.:
+- If the configured 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 write-failure visibility.
+
+2. Pair it with `file_flush_failures()` when diagnosing output-path instability.