chore: codebase audit, graceful shutdown, doc cleanup

[kacy]

summary

full codebase audit across all 6 crates (ember-server, ember-core, ember-protocol, ember-persistence, ember-cluster, ember-cli). 574 tests pass, clippy clean, zero doc warnings.

fixes from the audit:

• added graceful shutdown to the server — stops accepting connections on SIGINT/SIGTERM and drains active handlers before exiting (critical gap identified in audit)
• added fsync after AOF truncate header rewrite to ensure durability on crash-after-rewrite
• fixed all 56+ rustdoc warnings from unescaped angle brackets and square brackets in command doc comments
• fixed clippy range check style warnings in test assertions

documentation updates:

• updated all 6 crate READMEs to reflect the current feature set (hashes, sets, cluster commands were missing from ember-protocol; types incomplete in ember-core; graceful shutdown added to ember-server)
• removed stale "WIP" labels from cluster crate references

what was tested

• cargo test --workspace — 574 tests pass, 0 failures
• cargo clippy --workspace -- -D warnings — zero warnings
• cargo doc --workspace --no-deps — zero warnings (was 30+)
• cargo fmt --all --check — clean

design considerations

graceful shutdown: uses tokio::select! with biased to prioritize the shutdown signal over new accepts. drains by acquiring all semaphore permits (each connection releases its permit on exit). simple, no new dependencies, no broadcast channels needed.

AOF truncate fsync: the truncate method now calls sync_all() after flushing the fresh header. this closes a durability window where a crash between truncate and the next write could leave an unsynced header on disk.

doc comment style: switched from <angle> syntax (interpreted as HTML) to `backtick` syntax for parameters and \[escaped\] for optional flags. consistent across all 62 commands.