* fix(ec): off-by-one in nLargeBlockRows causes EC read corruption (#8947)
The nLargeBlockRows formula in locateOffset used (shardDatSize-1)/largeBlockLength,
which produces an off-by-one error when shardDatSize is an exact multiple of
largeBlockLength (e.g. a 30GB volume with 10 data shards = 3GB per shard).
This causes needles in the last large block row to be mislocated as small blocks,
reading from completely wrong shard positions and returning garbage data.
Fix: remove the -1 from locateOffset and only apply it in the ecdFileSize fallback
path (old volumes without datFileSize in .vif), where it's needed to handle the
ambiguous case conservatively.
Also fix ReadEcShardNeedle to pass offset=0 to ReadBytes, consistent with the
scrub path, since the bytes buffer already starts at position 0.
* fix: add volume context to EC read errors, remove contextless glog
The glog.Errorf in ReadBytes logged "entry not found" without any volume
ID, making it impossible to identify which volume was affected. Remove
this contextless log and instead add volume ID, needle ID, offset, and
size to the error returned from the EC read path.
The EC scrub callers already wrap errors with volume context.
Chris Lu