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.go

@@ -172,6 +172,10 @@ func (l *DiskLocation) loadExistingVolume(dirEntry os.DirEntry, needleMapKind Ne
 	// skip if ec volumes exists, but validate EC files first
 	if skipIfEcVolumesExists {
 		ecxFilePath := filepath.Join(l.IdxDirectory, volumeName+".ecx")
+		if !util.FileExists(ecxFilePath) && l.IdxDirectory != l.Directory {
+			// .ecx may have been created before -dir.idx was configured
+			ecxFilePath = filepath.Join(l.Directory, volumeName+".ecx")
+		}
 		if util.FileExists(ecxFilePath) {
 			// Validate EC volume: shard count, size consistency, and expected size vs .dat file
 			if !l.validateEcVolume(collection, vid) {