* fix(ec.decode): purge EC shards when volume is empty
When an EC volume has no live entries (all deleted), ec.decode should not generate an empty normal volume. Instead, treat decode as a no-op and allow shard purge to proceed cleanly.\n\nFixes: #7748
* chore: address PR review comments
* test: cover live EC index + avoid magic string
* chore: harden empty-EC handling
- Make shard cleanup best-effort (collect errors)\n- Remove unreachable EOF handling in HasLiveNeedles\n- Add empty ecx test case\n- Share no-live-entries substring between server/client\n
* perf: parallelize EC shard unmount/delete across locations
* refactor: combine unmount+delete into single goroutine per location
* refactor: use errors.Join for multi-error aggregation
* refactor: use existing ErrorWaitGroup for parallel execution
* fix: capture loop variables + clarify SuperBlockSize safety
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