feat: EXPIREAT, PEXPIREAT, GETSET, MSETNX, LPOP/RPOP count - #318

summary

adds six commands that fill in the remaining gaps in common string and list operations:

• EXPIREAT — set a key's expiry to an absolute unix timestamp in seconds
• PEXPIREAT — same but in milliseconds
• GETSET — atomically get the current value and replace it with a new one; clears any existing ttl
• MSETNX — set multiple key/value pairs atomically only if none of the keys already exist
• LPOP key [count] — optional count argument; without count returns a bulk string, with count returns an array
• RPOP key [count] — same semantics

what was tested

• 577 unit tests pass (cargo test -p emberkv-core)
• 412 protocol tests pass (cargo test -p ember-protocol)
• 12 new integration tests covering all six commands including edge cases:
  • expireat_sets_absolute_expiry, expireat_missing_key_returns_zero
  • pexpireat_sets_absolute_expiry_ms
  • getset_returns_old_value, getset_missing_key_returns_nil
  • msetnx_all_new_returns_one, msetnx_any_existing_returns_zero_and_no_changes
  • lpop_no_count_returns_bulk_string, lpop_count_returns_array, lpop_count_exceeding_list_returns_all
  • rpop_count_returns_array
• cargo clippy -D warnings clean on all affected crates

design notes

EXPIREAT/PEXPIREAT time model: ember stores expiry as a monotonic ms offset from process start, not unix epoch. a new unix_ms_to_monotonic_ms() function provides the inverse of the existing monotonic_to_unix_ms(), sharing a single ClockAnchor captured at startup so both conversions are coherent. EXPIREAT is persisted to AOF as Pexpireat (absolute unix ms); on recovery the remaining TTL is computed from wall time at that instant.

MSETNX fan-out: unlike MSET (which fans out individual Set requests), MSETNX needs to check all keys before writing any. we do two passes via route_multi: first Exists across all shards, then Set for each pair if all were absent. this is not atomic across shards in the same way redis isn't atomic across hash slots in cluster mode, but it is correct for single-node and cluster-mode when keys share a slot.

LPOP/RPOP count dispatch: count=None reuses the existing LPop/RPop shard variants (returning ShardResponse::Value) while count=Some(n) uses new LPopCount/RPopCount variants (returning ShardResponse::Array). this keeps the no-count path zero-overhead and avoids any conditional branching in the hot path.