chore: phase 4 audit fixes and documentation updates

[kacy]

Summary

post-phase-4 code audit with fixes and documentation updates:

code fixes:

• gossip event channel sends now log warnings when they fail (previously silently ignored with let _)
• removed semantically incorrect SlotRange::is_empty() method (always returned false)
• added #[allow(clippy::len_without_is_empty)] with proper invariant documentation

documentation updates:

• updated README.md to reflect phase 4 completion (574 tests, ~18k LOC)
• rewrote ember-cluster README with full feature list, command table, usage examples
• added invariant documentation to SlotRange

Tested

• cargo test --workspace - 574 tests pass
• cargo clippy --workspace - no warnings
• cargo fmt --all - formatted

Design considerations

what was intentionally deferred:

• raft log persistence (in-memory for now, noted in docs)
• server integration (modules complete, wiring deferred to phase 5)
• replication.rs (primary-replica sync)
• BLPOP/BRPOP (blocking ops require async coordination)

audit findings addressed:

• silent channel errors in gossip → now logged
• SlotRange semantic issue → removed is_empty(), documented invariant
• outdated READMEs → updated with current status