📝 补充runtime文档

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

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+---
+name: runtime-sink-drain-progress
+group: api
+category: runtime
+update-time: 20260707
+description: Return a structured generic drain progress snapshot for a RuntimeSink without collapsing queue and plain-file fallback semantics into one Int.
+key-word:
+    - runtime
+    - sink
+    - drain
+    - public
+---
+
+## Runtime-sink-drain-progress
+
+Return a structured generic drain progress snapshot for a `RuntimeSink`. This is the recommended truthful generic runtime drain helper when callers need bounded or full queue progress without relying on the compatibility `Int` returned by `drain()`.
+
+### Interface
+
+```moonbit
+pub fn RuntimeSink::drain_progress(
+  self : RuntimeSink,
+  max_items~ : Int = -1,
+) -> RuntimeSinkProgress {
+```
+
+#### input
+
+- `self : RuntimeSink` - Runtime sink whose generic drain progress should be observed.
+- `max_items : Int` - Optional upper bound on drained queued items. Negative values mean no explicit bound.
+
+#### output
+
+- `RuntimeSinkProgress` - Structured progress snapshot separating queue advancement from plain-file flush fallback steps.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- Queue-wrapped runtime sinks report consumed queued items through `queue_advanced_count` and honor `max_items`.
+- Plain file runtime sinks still follow the generic fallback path and report `FileSink::flush()` as `file_flush_step_count` `1` or `0`.
+- Plain console-style runtime sinks return a zeroed snapshot because they do not own a drainable queue here.
+- Queued file runtime sinks report queue advancement, while delayed file delivery failures still belong to file failure counters and `file_flush()`.
+- This helper preserves the old runtime behavior but makes the meaning explicit in the result shape.
+- `drain()` 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 Bounded Generic Drain Progress
+
+When code should observe one bounded drain step without mixing units:
+```moonbit
+let progress = sink.drain_progress(max_items=16)
+```
+
+In this example, queue progress and plain-file fallback stay separated.
+
+#### When Need A Full Manual Generic Drain Snapshot
+
+When support code should empty queued work and keep runtime-shape context:
+```moonbit
+let progress = sink.drain_progress()
+```
+
+In this example, the caller can inspect both the count and the runtime shape afterwards.
+
+### Error Case
+
+e.g.:
+- If the runtime sink is not queue-backed, the snapshot may stay zeroed or use the plain-file fallback field.
+
+- If callers want file-specific success semantics after queue delivery, `file_flush()` is the better API.
+
+### Notes
+
+1. Prefer `drain_progress()` over `drain()` in new generic runtime code.
+
+2. Use `pending_count()` together with this helper when remaining backlog must also be observed.