feat: LMPOP and ZMPOP commands

[kacy]

summary

• adds LMPOP numkeys key [key ...] LEFT|RIGHT [COUNT n] for atomic multi-key list pop
• adds ZMPOP numkeys key [key ...] MIN|MAX [COUNT n] for atomic multi-key sorted set pop
• both commands scan keys left-to-right, popping from the first non-empty collection
• optional COUNT defaults to 1; returns [key_name, [elements]] or nil when all keys empty
• AOF: LMPOP replays as individual LPOP/RPOP records; ZMPOP replays as ZREM

tested

• 5 unit tests for lpop_count/rpop_count in keyspace (count, capped-at-size, missing key, wrong type)
• 6 integration tests: lmpop basic (skips empty keys), lmpop COUNT, lmpop all-empty nil, zmpop MIN, zmpop MAX COUNT, zmpop all-empty nil
• cargo clippy -D warnings clean on ember-protocol, emberkv-core, ember-server
• 565 unit tests pass

design notes

execute.rs drives the key-scan loop: for each key, it sends a LmpopSingle/ZmpopSingle sub-request to the owning shard and returns immediately on the first non-empty result. this keeps all the routing logic in execute.rs and the shard dispatch simple.