📝 document file sink reopen methods

docs/api/file-sink-reopen-truncate.md

@@ -0,0 +1,74 @@
+---
+name: file-sink-reopen-truncate
+group: api
+category: sink
+update-time: 20260613
+description: Reopen a FileSink in truncate mode.
+key-word:
+    - file
+    - sink
+    - reopen
+    - public
+---
+
+## File-sink-reopen-truncate
+
+Reopen a `FileSink` in truncate mode. This helper is the explicit truncate-oriented recovery or reset shortcut on the direct sink.
+
+### Interface
+
+```moonbit
+pub fn FileSink::reopen_truncate(self : FileSink) -> Bool {
+```
+
+#### input
+
+- `self : FileSink` - File sink that should be reopened in truncate mode.
+
+#### output
+
+- `Bool` - Whether reopen succeeded.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- This helper is a specialized shortcut over `reopen(append=Some(false))`.
+- Reopen behavior is fixed to truncate mode.
+- The stored append policy is updated to `false` as part of the reopen path.
+- On failure, the sink remains unavailable and `open_failures` is incremented.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need A Fresh Output File
+
+When a direct runtime file should be reopened from an empty state:
+```moonbit
+ignore(sink.reopen_truncate())
+```
+
+In this example, reopen behavior truncates the file before future writes continue.
+
+#### When Want An Explicit Truncate Shortcut
+
+When code should make destructive reopen intent obvious:
+```moonbit
+let ok = sink.reopen_truncate()
+```
+
+In this example, the call site expresses reset-style reopen behavior directly.
+
+### Error Case
+
+e.g.:
+- If callers want to preserve existing file content, `reopen_append()` is the correct API.
+
+- If reopen fails, the helper returns `false` and the sink stays unavailable.
+
+### Notes
+
+1. Use this helper when starting from a fresh file is intentional.
+
+2. Truncate-mode reopen is a stronger action than generic reopen recovery.