📝 Add configured logger file policy API docs

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

@@ -0,0 +1,75 @@
+---
+name: configured-logger-file-policy
+group: api
+category: runtime
+update-time: 20260512
+description: Read the current runtime file policy from a configured file-backed logger.
+key-word:
+    - logger
+    - runtime
+    - file
+    - public
+---
+
+## Configured-logger-file-policy
+
+Read the current runtime file policy from a `ConfiguredLogger`. This helper exposes the active append, auto-flush, and rotation settings as one policy object.
+
+### Interface
+
+```moonbit
+pub fn ConfiguredLogger::file_policy(self : ConfiguredLogger) -> FileSinkPolicy {}
+```
+
+#### input
+
+- `self : ConfiguredLogger` - Config-driven runtime logger whose current file policy should be inspected.
+
+#### output
+
+- `FileSinkPolicy` - Current runtime file policy.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- File-backed sinks return their current runtime file policy.
+- Queued file sinks forward the policy from the wrapped file sink.
+- Non-file sinks return a neutral fallback policy value.
+- This helper is broader than `file_append_mode()` or `file_auto_flush()` because it returns the whole policy object.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Full Runtime Policy Visibility
+
+When diagnostics should inspect the active file policy as one object:
+```moonbit
+let policy = logger.file_policy()
+```
+
+In this example, append, flush, and rotation settings are read together.
+
+#### When Compare Current And Default Policy
+
+When runtime drift from defaults should be inspected explicitly:
+```moonbit
+let current = logger.file_policy()
+let defaults = logger.file_default_policy()
+```
+
+In this example, callers can compare current runtime settings with the initial policy snapshot.
+
+### Error Case
+
+e.g.:
+- If the configured sink is not file-backed, the return value is a neutral fallback policy rather than a real active file policy.
+
+- If callers only need one field from the policy, a narrower helper may be simpler.
+
+### Notes
+
+1. Use this helper when file policy should be handled as one object.
+
+2. Pair it with `file_set_policy(...)` for roundtrip-style policy management.