ec: fall back to data dir when ecx file not found in idx dir (#8541)

* ec: fall back to data dir when ecx file not found in idx dir (#8540)

When -dir.idx is configured after EC encoding, the .ecx/.ecj files
remain in the data directory. NewEcVolume now falls back to the data
directory when the index file is not found in dirIdx.

* ec: add fallback logging and improved error message for ecx lookup

* ec: preserve configured dirIdx, track actual ecx location separately

The previous fallback set ev.dirIdx = dir when finding .ecx in the data
directory, which corrupted IndexBaseFileName() for future writes (e.g.,
WriteIdxFileFromEcIndex during EC-to-volume conversion would write the
.idx file to the data directory instead of the configured index directory).

Introduce ecxActualDir to track where .ecx/.ecj were actually found,
used only by FileName() for cleanup/destroy. IndexBaseFileName() continues
to use the configured dirIdx for new file creation.

* ec: check both idx and data dirs for .ecx in all cleanup and lookup paths

When -dir.idx is configured after EC encoding, .ecx/.ecj files may
reside in the data directory. Several code paths only checked
l.IdxDirectory, causing them to miss these files:

- removeEcVolumeFiles: now removes .ecx/.ecj from both directories
- loadExistingVolume: ecx existence check falls back to data dir
- deleteEcShardIdsForEachLocation: ecx existence check and cleanup
  both cover the data directory
- VolumeEcShardsRebuild: ecx lookup falls back to data directory
  so RebuildEcxFile operates on the correct file

weed/storage/disk_location_ec.go

@@ -476,6 +476,11 @@ func (l *DiskLocation) removeEcVolumeFiles(collection string, vid needle.VolumeI
 	// EC loading for incomplete/missing shards on next startup
 	removeFile(indexBaseFileName+".ecx", "EC index file")
 	removeFile(indexBaseFileName+".ecj", "EC journal file")
+	// Also try the data directory in case .ecx/.ecj were created before -dir.idx was configured
+	if l.IdxDirectory != l.Directory {
+		removeFile(baseFileName+".ecx", "EC index file (fallback)")
+		removeFile(baseFileName+".ecj", "EC journal file (fallback)")
+	}
 
 	// Remove all EC shard files (.ec00 ~ .ec31) from data directory
 	// Use MaxShardCount (32) to support custom EC ratios