TigerStyle: add verify_invariants() to CommandExecutor

[nerdsane]

Context

Found during code review of bitmap implementation (commit e9be18b).

Problem

CommandExecutor has no verify_invariants() method. Data structure types like SDS and sorted sets have them, but the executor itself does not verify its aggregate state after mutations. This means:

• data and expirations maps could diverge silently
• access_times could contain stale entries
• key_count field could drift from data.len()

Suggested Invariants

impl CommandExecutor {
    #[cfg(debug_assertions)]
    fn verify_invariants(&self) {
        // Every key with expiration must exist in data
        for key in self.expirations.keys() {
            debug_assert!(self.data.contains_key(key));
        }
        // access_times should be subset of data keys
        // (with allowance for recently-expired keys)
    }
}

Call after every mutation in debug builds.

Severity

Low — design improvement, pre-existing gap across all executor ops.

Found By

TigerStyle review agent (Design observation)