fix(cold): fix for spurious test failure

[Fraser999]

Summary

Fixes a spurious failure in fairness_write_serves_before_later_readers (~3% flake rate locally).

The test compared Notify signals fired from caller-side wrappers after truncate_above().await / get_latest_block().await returned. Both signals fire after the writer drops the drain barrier - which is the same instant later readers' permits become available - so the wrapper-task signaling order is racy under multi-thread scheduling even though the underlying semaphore is FIFO-fair.

Fix

Observe ordering inside the backend instead of via post-completion signals. GatedBackend now records each instrumented method into an events: Vec<BackendOp> log at a well-defined point in the call:

• GetLatestBlock is recorded after the read passes the gate, before its read_sem permit drops.
• TruncateAbove is recorded after the inner write returns, while the unified handle still holds the drain barrier - so the entry in the log proves the write completed before any later reader could run.

The semaphore forces a strict order on those entries: 64 saturating reads, then the write, then 64 later reads. The test asserts that shape directly, with no Notify and no select! racing the post-drain wake-up window.

Note: BackendOp is an enum so labels are type-safe and future tests get compile-time matching. Only the two methods this test needs are instrumented today; extend BackendOp and the relevant impl when adding tests that need more.

Test plan

• cargo test -p signet-cold --test concurrency - all 7 tests pass
• Stress: 100/100 runs of fairness_write_serves_before_later_readers (vs ~29/30 before)
• cargo +nightly fmt -- --check
• cargo clippy -p signet-cold --all-features --all-targets
• cargo clippy -p signet-cold --no-default-features --all-targets

[Fraser999]

• fix(cold): fix for spurious test failure #69 👈 (View in Graphite)
• main

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[Evalir]

lgtm. appreciate the thoroughness on the doc

[Evalir]

this makes sense to me