Restore counter-based CAS semantics in segcache

[thinkingfish]

Summary

The hashtable rewrite (#10) replaced the per-bucket CAS counter with the item's packed location bits as the CAS token. This regressed memcache CAS semantics for downstream consumers: the cas stored integration test receives EXISTS where STORED is expected, which is why the downstream segcache git dependency has stayed pinned at 5d1d870.

Two concrete problems with location-derived tokens:

• Tokens are no longer small, deterministic, monotonically increasing integers (1, 2, 3, …), which downstream memcache integration tests depend on.
• Tokens silently change whenever an item is relocated (e.g. by merge eviction copying items to a new segment), invalidating outstanding tokens even though the value never changed.

Fix

Restore the original semantics with a per-bucket AtomicU32 CAS counter in MultiChoiceHashtable, indexed by a key's primary (first-choice) bucket — the new symmetric-slot bucket design has no metadata word to hold it, so the counters live in a parallel array (4 bytes per 64-byte bucket):

• incremented on every successful hashtable insert
• incremented again on a successful CAS check, matching the original double-bump behavior of try_update_cas + insert
• left untouched by relocation (cas_location), removal, and ghost conversion
• reset by clear()

Item::cas() and Segcache::cas() now read this counter instead of the location bits. The cas() lookup also counts as a hit (frequency update) again, as it did in the original implementation.

Testing

• New test_cas_value_counter unit test covers the counter lifecycle at the hashtable level (insert bumps, relocation/removal don't, clear resets).
• New cas_deterministic end-to-end test pins the exact token sequence the original implementation produced (1 after first set, 2 after overwrite, 4 after a successful CAS).
• cargo test --workspace, cargo test -p segcache --features loom -- loom (CI invocation, 8/8 models pass), and cargo clippy --all-targets are all green.

After this lands, the downstream dependency bump past c2f43b8 should no longer regress the cas stored test.

https://claude.ai/code/session_01B7MKPvEtTUmRX996r1uKku

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