📝 document file sink policy state methods

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

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+---
+name: file-sink-default-policy
+group: api
+category: sink
+update-time: 20260613
+description: Read the initial default file policy captured by a FileSink.
+key-word:
+    - file
+    - sink
+    - policy
+    - public
+---
+
+## File-sink-default-policy
+
+Read the initial default file policy captured by a `FileSink`. This helper exposes the baseline file policy recorded when the sink was created.
+
+### Interface
+
+```moonbit
+pub fn FileSink::default_policy(self : FileSink) -> FileSinkPolicy {
+```
+
+#### input
+
+- `self : FileSink` - File sink whose default file policy should be inspected.
+
+#### output
+
+- `FileSinkPolicy` - Initial default file policy.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- The returned policy is built from the sink's stored default append, auto-flush, and rotation values.
+- This helper is useful when callers need to compare runtime drift or restore defaults later.
+- It is observation-only and does not mutate sink state.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Baseline Policy Visibility
+
+When diagnostics should show the original file policy separately from the live one:
+```moonbit
+let defaults = sink.default_policy()
+```
+
+In this example, the direct file sink exposes its original file policy snapshot.
+
+#### When Prepare For Policy Reset Logic
+
+When tooling should capture or compare default settings explicitly:
+```moonbit
+let original = sink.default_policy()
+```
+
+In this example, callers can reason about factory policy separately from runtime changes.
+
+### Error Case
+
+e.g.:
+- If callers only need to know whether runtime drift exists, `policy_matches_default()` is the simpler API.
+
+- This helper does not say whether the current sink handle is available.
+
+### Notes
+
+1. Use this helper when the original file policy matters operationally.
+
+2. It complements `policy()` and `reset_policy()`.