rust volume: group-commit fsync uploads on the direct write path (#11860)

* rust volume: group-commit fsync uploads on the direct write path

Each fsync=true upload used to run its own .dat/.idx sync when the write
queue was off, which is the default. Route durable uploads through the
per-volume write workers so a batch shares one sync, matching Go's
writeNeedle2: non-fsync writes, and writes while the server is stopping,
stay on the synchronous path. The queue is now always constructed and
SEAWEED_WRITE_QUEUE is gone.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* rust volume: document post_handler write routing

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

---------

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

seaweed-volume/src/server/volume_server.rs

@@ -77,7 +77,9 @@ pub struct VolumeServerState {
     pub pre_stop_seconds: u32,
     /// Notify heartbeat to send an immediate update when volume state changes.
     pub volume_state_notify: tokio::sync::Notify,
-    /// Optional batched write queue for improved throughput under load.
+    /// Per-volume write workers that group-commit durable (fsync) uploads.
+    /// Set once at startup in production; a test state may leave it unset,
+    /// which sends every write down the synchronous path.
     pub write_queue: std::sync::OnceLock<WriteQueue>,
     /// Read mode: local, proxy, or redirect for non-local volumes.
     pub read_mode: ReadMode,