ec: skip physically near-full disks when placing EC shards (#10167)

EC placement scored destinations purely by free EC shard slots (derived from
maxVolumeCount) and shard counts, blind to real disk fullness — the same defect
as volume balancing. A disk that is physically full but still shows free EC slots
kept being chosen, and EC shard bytes are captured by statfs free space yet not
by any slot accounting, so the slot math is exactly the metric that can't see EC
fullness.

Treat a disk at/above 90% physical usage as having zero free EC slots at
snapshot-build time, so every existing freeSlots>0 placement predicate excludes
it. Applied in all three snapshot builders (shell countFreeShardSlots, the shared
ecbalancer FromActiveTopology, and the worker ec_balance buildBalancerTopology)
via the shared balancer.DiskTooFullAfter gate. Servers not reporting disk bytes
fall back to slot-only behavior. ec.rebuild recovery is left ungated so shard
recovery can still complete onto fuller disks.

weed/shell/command_ec_common.go

@@ -19,6 +19,7 @@ import (
 	"github.com/seaweedfs/seaweedfs/weed/storage/needle"
 	"github.com/seaweedfs/seaweedfs/weed/storage/super_block"
 	"github.com/seaweedfs/seaweedfs/weed/storage/types"
+	"github.com/seaweedfs/seaweedfs/weed/topology/balancer"
 	"google.golang.org/grpc"
 )
 
@@ -469,6 +470,13 @@ func countFreeShardSlots(dn *master_pb.DataNodeInfo, diskType types.DiskType) (c
 		return 0
 	}
 
+	// A physically near-full disk has no room for more EC shards regardless of
+	// slot math (an over-set maxVolumeCount hides real fullness; statfs free bytes
+	// already include EC shard files). No-opinion when the server reports no bytes.
+	if balancer.DiskTooFullAfter(diskInfo.DiskTotalBytes, diskInfo.DiskFreeBytes, 0, balancer.DefaultMaxDiskUsagePercent) {
+		return 0
+	}
+
 	slots := int(diskInfo.MaxVolumeCount-diskInfo.VolumeCount)*erasure_coding.DataShardsCount - countShards(diskInfo.EcShardInfos)
 	if slots < 0 {
 		return 0