📝 Add configured logger file runtime API docs

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

@@ -0,0 +1,74 @@
+---
+name: configured-logger-file-path
+group: api
+category: runtime
+update-time: 20260512
+description: Read the effective file path used by the configured runtime logger when it is file-backed.
+key-word:
+    - logger
+    - runtime
+    - file
+    - public
+---
+
+## Configured-logger-file-path
+
+Read the effective file path used by a `ConfiguredLogger`. This helper is useful for diagnostics, support output, and confirming which file-backed runtime sink is active.
+
+### Interface
+
+```moonbit
+pub fn ConfiguredLogger::file_path(self : ConfiguredLogger) -> String {}
+```
+
+#### input
+
+- `self : ConfiguredLogger` - Config-driven runtime logger whose file path should be inspected.
+
+#### output
+
+- `String` - Effective runtime file path, or an empty string for non-file sinks.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- File-backed sinks return their current file path.
+- Queued file sinks forward the wrapped file sink path.
+- Non-file sinks return an empty string.
+- This helper is observation-only and does not modify file state.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Runtime Path Diagnostics
+
+When diagnostics should show which file is active:
+```moonbit
+println(logger.file_path())
+```
+
+In this example, operators can verify the effective destination path directly.
+
+#### When Build Support Output
+
+When application state dumps should include file destination info:
+```moonbit
+let path = logger.file_path()
+```
+
+In this example, the path can be surfaced without reading broader runtime state.
+
+### Error Case
+
+e.g.:
+- If the configured sink is not file-backed, the method returns an empty string.
+
+- If callers need richer file status than just the path, `file_state()` or `file_runtime_state()` is the better API.
+
+### Notes
+
+1. Use this helper for direct runtime path visibility.
+
+2. Empty string usually means the configured sink is not file-backed.