Replaces the test-time GLOBAL_STATE_LOCK approach (which was just disguised
single-threading) with proper test isolation: each test thread gets its own
SnapshotState via a thread_local!.
Pattern matches crdt_state::CRDT_STATE_TL — production keeps the global
OnceLock; tests get a per-thread OnceLock that's accessed through a
snapshot_state() helper. The unsafe `&*ptr` cast to 'static is safe because
the thread_local lives as long as the spawning test thread.
The race: latest_snapshot_available_after_compaction captured at_seq from a
freshly-generated snapshot, then asserted it equalled SNAPSHOT_STATE's
latest.at_seq. With shared SNAPSHOT_STATE, another test thread's
apply_compaction could overwrite latest_snapshot between capture and read.
Per-thread state eliminates the race at its source.
ALL_OPS / VECTOR_CLOCK stay shared — the tests don't assert on absolute
counts, only on (this-thread's at_seq) == (this-thread's latest.at_seq).
5 consecutive default-parallel `cargo test --bin huskies` runs all green
at 2636/2636.
dave
7408cc5b4b