📝 Add configured logger file policy API docs

docs/api/configured-logger-file-default-policy.md

@@ -0,0 +1,74 @@
+---
+name: configured-logger-file-default-policy
+group: api
+category: runtime
+update-time: 20260512
+description: Read the initial default file policy associated with a configured file-backed logger.
+key-word:
+    - logger
+    - runtime
+    - file
+    - public
+---
+
+## Configured-logger-file-default-policy
+
+Read the initial default file policy associated with a `ConfiguredLogger`. This helper exposes the baseline file policy captured when the runtime sink was created.
+
+### Interface
+
+```moonbit
+pub fn ConfiguredLogger::file_default_policy(self : ConfiguredLogger) -> FileSinkPolicy {}
+```
+
+#### input
+
+- `self : ConfiguredLogger` - Config-driven runtime logger whose default file policy should be inspected.
+
+#### output
+
+- `FileSinkPolicy` - Initial default file policy.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- File-backed sinks return the default policy captured at creation time.
+- Queued file sinks forward the default policy from the wrapped file sink.
+- Non-file sinks return a neutral fallback policy value.
+- This helper is useful when callers need to compare runtime drift or restore defaults later.
+
+### 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 = logger.file_default_policy()
+```
+
+In this example, the configured logger exposes its original file policy snapshot.
+
+#### When Prepare For Policy Reset Logic
+
+When tooling should capture or compare default settings explicitly:
+```moonbit
+let original = logger.file_default_policy()
+```
+
+In this example, callers can reason about “factory” file policy separately from runtime changes.
+
+### Error Case
+
+e.g.:
+- If the configured sink is not file-backed, the return value is a neutral fallback policy.
+
+- If callers only need to know whether runtime drift exists, `file_policy_matches_default()` is the simpler API.
+
+### Notes
+
+1. Use this helper when the original file policy matters operationally.
+
+2. It complements `file_policy()` and `file_reset_policy()`.