refactor: use monotonic clock for state timing

[costela]

What

State timing (Circuit.openedAt and SlidingWindowBreaker.currentStart) now uses the monotonic clock instead of wall-clock micros.

Why

Both previously stored time.Now().UnixMicro() and measured elapsed time with time.Since(time.UnixMicro(...)), which strips the monotonic reading and falls back to wall-clock subtraction. A wall-clock jump (NTP step, manual change) could pin a circuit open past its halfOpenDelay, or skip the open period entirely — exactly the conditions under which a resilience library is most likely to be running.

How

• A package-level monotonic reference start = time.Now() captured once at load.
• nowNanos() / sinceNanos() helpers store and compare elapsed nanoseconds against it.
• Keeps the lock-free single-int64 atomic state; transitions are now immune to wall-clock jumps.
• openedAt/currentStart are now monotonic-since-start nanoseconds (unexported state, not wall-clock meaningful).

Notes

• Pure behavior-preserving refactor under normal clocks; no API change.
• The pre-existing EWMA test flakiness (TestBreaker_Observe_State/...EWMA flaky) is unrelated and addressed in a separate PR.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR refactors internal state timing for Circuit.openedAt and SlidingWindowBreaker.currentStart to use a monotonic time source (elapsed nanoseconds since a package-load reference) so state transitions aren’t affected by wall-clock jumps.

Changes:

• Replace wall-clock microsecond timestamps with monotonic “nanos since start” timestamps for circuit state timing.
• Add start, nowNanos(), and sinceNanos() helpers to store/compare monotonic elapsed time.
• Update tests to simulate half-open timing using the new monotonic timestamp scheme.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
hoglet.go	Introduces monotonic time helpers and migrates circuit open/half-open timing to monotonic nanos.
breaker.go	Migrates sliding-window timing from wall-clock micros to monotonic nanos.
hoglet_test.go	Updates test setup to simulate half-open passage of time using monotonic timestamps.

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