fix: address PR 15 review gaps — verify_invariants, validation, dead code

[nerdsane]

Summary

Fixes 5 gaps identified by domain expert review of PR #15 (memory optimizations).

• CommandExecutor::verify_invariants() — New TigerStyle method checking 3 invariants: orphaned expirations, stale expired keys, empty collections. Called after every eviction in debug builds.
• ConnectionPoolConfig validation — max_connections > 0 and buffer_pool_size > 0 bounds added to PerformanceConfig::validate()
• Remove dead key_count field — Was initialized to 0, never updated, never read, had #[allow(dead_code)]
• Vec::with_capacity() — Pre-allocate expired_keys vec in both eviction methods
• 4 new tests — ConnectionPoolConfig defaults, TOML parsing, partial TOML, validation errors

What verify_invariants() catches

This closes the gap where a bug like "LPOP forgets to clean up empty list" would only be caught if LLEN happened to be called on that exact key later. Now it's caught immediately after every eviction cycle.

// Invariant 1: every key in expirations exists in data
// Invariant 2: no expired keys remain after eviction
// Invariant 3: no empty collections in data

Test plan

• 9 executor DST tests pass (verify_invariants fires across 100+ seeds × 1000+ ops)
• 8 perf_config tests pass (including 4 new)
• cargo clippy --release -p redis-sim -- -D warnings — 0 warnings

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