* erasure_coding: WriteDatFile takes the encode-time dat size for the shard block layout
* volume server: derive EC decode layout from the encode-time dat size, not the live extent
* erasure_coding: test decode after tail deletions shrink the live extent below a large-block row
* seaweed-volume: write_dat_file_from_shards takes the encode-time dat size for the shard block layout
* seaweed-volume: derive EC decode layout from the encode-time dat size, not the live extent
* seaweed-volume: test decode after tail deletions shrink the live extent below a large-block row
* erasure_coding: reject decoding with no data shards
* worker: record the encode-time dat size in the .vif
* erasure_coding: fall back to the shard-derived layout only when the encode-time dat size is missing
* erasure_coding: reject an ambiguous shard-derived block layout
* seaweed-volume: fall back to the shard-derived layout only when the encode-time dat size is missing
* seaweed-volume: reject an ambiguous shard-derived block layout