feat: Phase 4A CP4a -- simulated master, assignment sequence tests, BlockVolumeStatus

Add SimulatedMaster test helper + 20 assignment sequence tests (8 sequence,
5 failover, 5 adversarial, 2 status). Add BlockVolumeStatus struct and
Status() method. Includes QA test files for CP1-CP4a. 940 total unit tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

weed/storage/blockvol/group_commit.go

@@ -129,7 +129,7 @@ func (gc *GroupCommitter) Run() {
 		}
 
 		// Perform fsync with panic recovery. A panic in syncFunc must
-		// not leave waiters hung — notify them and drain stragglers.
+		// not leave waiters hung --notify them and drain stragglers.
 		err := gc.callSyncFunc()
 		gc.syncCount.Add(1)
 		if err == nil && gc.postSyncCheck != nil {