feat: RANDOMKEY, TOUCH, SORT commands

[kacy]

summary

adds three convenience commands from the audit backlog (#20) that client libraries and Redis-compatible tools occasionally probe for. none touch the hot path — zero impact on GET/SET throughput.

• RANDOMKEY: returns a random key from the database, or nil if empty. broadcasts across all shards via engine.broadcast() and picks a random result using reservoir sampling.
• TOUCH key [key ...]: updates last access time for given keys, returns count that existed. reuses the same multi_key_bool + route_multi fan-out pattern as DEL/EXISTS.
• SORT key [ASC|DESC] [ALPHA] [LIMIT offset count] [STORE dest]: sorts elements from a list, set, or sorted set. numeric sort by default, lexicographic with ALPHA. STORE variant does two-phase sort + rpush to destination key. BY/GET patterns intentionally deferred — rarely used and adds significant complexity.

~80 LOC new logic, ~40 LOC modified across 6 files (plus tests and CLI metadata).

what was tested

• cargo fmt --all --check — clean
• cargo clippy --workspace -- -D warnings — clean
• cargo test -p ember-protocol — 406 passed
• cargo test -p emberkv-core — 401 passed
• cargo test -p ember-server — 134 passed
• cargo test -p ember-integration-tests — 79 passed
• new parser tests: RANDOMKEY (2), TOUCH (3), SORT (7 — basic, desc+alpha, limit, store, no-args, is_write with/without store)
• new keyspace tests: random_key empty/non-empty, touch exists/missing, sort list numeric/alpha-desc/limit, sort set alpha, sort missing key, sort wrong type

design considerations

• SORT without STORE routes through route! macro for batch dispatch efficiency; SORT with STORE falls through to execute() for two-phase inline handling (sort on source shard → del+rpush on dest shard)
• TOUCH is categorized as READ | KEYSPACE | FAST (matching Redis), not as a write — it only updates LRU metadata, not user-visible state
• concurrent mode returns an error for SORT (only supports strings) and uses exists() for TOUCH (no last_access tracking in DashMap entries)
• cluster slot checks: TOUCH added to multi-key crossslot group; SORT without STORE added to single-key slot check; SORT with STORE gets crossslot validation between source and destination keys