* 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
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