perf: move version field to lazy side table

[kacy]

summary

moves the version: u64 field from every Entry in the keyspace to a lazily-populated side table (AHashMap<CompactString, u64>) on Keyspace. this field was only meaningful for WATCH/EXEC optimistic locking (<1% of workloads) but consumed 8 bytes on every single entry.

• key_version() now inserts into the side table on first call (simulating WATCH registration)
• bump_version() only bumps the version if the key is already tracked — on the hot path this is a fast hash-miss on an empty map
• version entries are cleaned up on delete, expire, evict, flush, and clear
• ENTRY_OVERHEAD reduced from 128 to 120 bytes

estimated savings: ~8 bytes per key in sharded mode (208 → ~200 B/key).

what was tested

• all 507 unit tests pass (cargo test -p emberkv-core)
• full workspace builds and passes all tests (cargo test --all)
• cargo clippy --all clean (no new warnings)
• entry_overhead_not_too_small test validates the new constant against actual struct sizes

design considerations

the alternative was keeping version on Entry but using a smaller type (u32). that only saves 4 bytes with alignment and still writes 4 bytes on every mutation. the side table approach saves 8 bytes per entry and eliminates the unconditional write on the hot path entirely — mutations now only do a hash lookup against self.versions which is almost always empty (fast miss).

the tradeoff is that WATCH/EXEC now has slightly different bookkeeping (lazy population on first key_version() call), but the semantics are identical: WATCH captures a snapshot version, EXEC detects changes.