fix: WAL rotation skipped for all nodes due to non-blocking send on small channel

RotateWALFiles used a non-blocking send (select/default) on rotation
channels buffered at 64. With thousands of nodes and few shards, the
channel fills instantly and nearly all hosts are skipped, leaving WAL
files unrotated indefinitely.

Replace with a blocking send using a shared 2-minute deadline so the
checkpoint goroutine waits for the staging goroutine to drain the
channel instead of immediately giving up.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Entire-Checkpoint: a1ec897216fa

pkg/metricstore/walCheckpoint.go

@@ -358,6 +358,7 @@ func RotateWALFiles(hostDirs []string) {
 	if walShardRotateChs == nil || walShuttingDown.Load() {
 		return
 	}
+	deadline := time.After(2 * time.Minute)
 	dones := make([]chan struct{}, 0, len(hostDirs))
 	for _, dir := range hostDirs {
 		done := make(chan struct{})
@@ -365,16 +366,18 @@ func RotateWALFiles(hostDirs []string) {
 		select {
 		case walShardRotateChs[shard] <- walRotateReq{hostDir: dir, done: done}:
 			dones = append(dones, done)
-		default:
-			// Channel full or goroutine not consuming — skip this host.
-			cclog.Warnf("[METRICSTORE]> WAL rotation skipped for %s (channel full)", dir)
+		case <-deadline:
+			cclog.Warnf("[METRICSTORE]> WAL rotation send timed out, %d of %d hosts remaining",
+				len(hostDirs)-len(dones), len(hostDirs))
+			goto waitDones
 		}
 	}
+waitDones:
 	for _, done := range dones {
 		select {
 		case <-done:
 		case <-time.After(30 * time.Second):
-			cclog.Warn("[METRICSTORE]> WAL rotation timed out, continuing")
+			cclog.Warn("[METRICSTORE]> WAL rotation completion timed out, continuing")
 			return
 		}
 	}