📝 document file sink counters and resets

docs/api/file-sink-reset-policy.md

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+---
+name: file-sink-reset-policy
+group: api
+category: sink
+update-time: 20260613
+description: Reset the runtime file policy of a FileSink back to its original defaults.
+key-word:
+    - file
+    - sink
+    - reset
+    - public
+---
+
+## File-sink-reset-policy
+
+Reset the runtime file policy of a `FileSink` back to its original defaults. This helper restores append, auto-flush, and rotation policy together on the direct sink.
+
+### Interface
+
+```moonbit
+pub fn FileSink::reset_policy(self : FileSink) -> Unit {
+```
+
+#### input
+
+- `self : FileSink` - File sink whose file policy should be restored.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- This helper restores the sink's stored default append, auto-flush, and rotation settings.
+- It is the inverse of runtime policy drift, not a generic reopen or flush action.
+- It changes policy only and does not reopen the sink automatically.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need To Undo Runtime Policy Changes
+
+When direct file policy should return to the original configured defaults:
+```moonbit
+sink.reset_policy()
+```
+
+In this example, append, auto-flush, and rotation settings are restored together.
+
+#### When Use Drift-aware Recovery
+
+When reset should only happen after runtime changes:
+```moonbit
+if !sink.policy_matches_default() {
+  sink.reset_policy()
+}
+```
+
+In this example, reset is only applied when runtime policy has diverged.
+
+### Error Case
+
+e.g.:
+- If callers need to restore only one setting, a narrower setter may be more appropriate than a full policy reset.
+
+- This helper does not reopen the file or prove current availability.
+
+### Notes
+
+1. Use this helper when the original bundled file policy should be restored intact.
+
+2. It pairs naturally with `policy_matches_default()` and `default_policy()`.