📝 补充runtime文档

docs/api/runtime-sink-flush-progress.md

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+---
+name: runtime-sink-flush-progress
+group: api
+category: runtime
+update-time: 20260707
+description: Return a structured generic flush progress snapshot for a RuntimeSink without collapsing queue and plain-file fallback semantics into one Int.
+key-word:
+    - runtime
+    - sink
+    - flush
+    - public
+---
+
+## Runtime-sink-flush-progress
+
+Return a structured generic flush progress snapshot for a `RuntimeSink`. This is the recommended truthful generic runtime flush helper when callers need to observe progress without relying on the compatibility `Int` returned by `flush()`.
+
+### Interface
+
+```moonbit
+pub fn RuntimeSink::flush_progress(self : RuntimeSink) -> RuntimeSinkProgress {
+```
+
+#### input
+
+- `self : RuntimeSink` - Runtime sink whose generic flush progress should be observed.
+
+#### output
+
+- `RuntimeSinkProgress` - Structured progress snapshot separating queue advancement from plain-file flush fallback steps.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- Plain console-style runtime sinks return a zeroed snapshot with `queue_backed=false` and `file_backed=false`.
+- Plain file runtime sinks call `FileSink::flush()` and report the fallback result through `file_flush_step_count` as `1` or `0`.
+- Queue-wrapped runtime sinks report consumed queued items through `queue_advanced_count`.
+- Queued file runtime sinks still report generic progress as queue advancement, while `file_backed=true` marks that the queue drains into a file sink.
+- This helper does not convert different runtime-shape behaviors into one ambiguous compatibility `Int`.
+- `flush()` remains available as the legacy convenience wrapper that sums the fields back into one count.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Recommended Generic Flush Progress
+
+When direct runtime code should inspect flush progress explicitly:
+```moonbit
+let progress = sink.flush_progress()
+```
+
+In this example, the caller gets a structured snapshot instead of a mixed `Int`.
+
+#### When Need Queue-aware Branching During Flush
+
+When generic runtime support code should react differently for queue-backed and plain-file sinks:
+```moonbit
+let progress = sink.flush_progress()
+if progress.queue_backed {
+  println(progress.queue_advanced_count)
+}
+```
+
+In this example, queue advancement is explicit and does not need to be inferred from runtime shape.
+
+### Error Case
+
+e.g.:
+- If the runtime sink shape has no flushable state, the snapshot simply reports zero counts.
+
+- If callers need file-specific success semantics rather than generic runtime progress, `file_flush()` is the better API.
+
+### Notes
+
+1. Prefer `flush_progress()` over `flush()` in new generic runtime code.
+
+2. Use `flush()` only when older compatibility code still expects one numeric count.