* shell: move ErrorWaitGroup to weed/util
* shell: remove unused CandidateEcNode and EcRack types
* ec: extract EC orchestration logic from weed/shell into weed/ec
Move the EC node/topology model, balance engine, encode pipeline, decode
pipeline, and rebuild engine into a new weed/ec package so shell commands
and maintenance workers can share the logic. Shell commands keep flag
parsing and delegate through a small ec.Env (dial option, topology fetch,
volume locations, lock check). Tests move along with the code.
* shell: remove unused proportional-rebalance type stubs
* ec: move scrub, replication check, and shard unmount engines into weed/ec
* worker: share the EC generation-aware shard counter from weed/ec
* ec: gofmt
* shell: drop EC aliases with no remaining callers
* ec: guard a missing topology hook and nil disk entries in topology helpers
* ec: drop trailing newlines from decode error strings
* ec: re-check the shell lock before applying shard unmounts
* shell: trim -node entries in ec.scrub
