feat: defer actor restarts with exponential backoff

[joshua-temple]

Adds the Backoff supervision decision: instead of restarting a failed actor immediately, the supervisor schedules the re-spawn after an exponentially growing delay (initial * factor^n, capped, per the per-src budget set with WithBackoff) and the host applies due restarts by calling Supervisor.Tick. Time is read through an injected state.Clock (WithClock, default the system clock), so backoff is fully deterministic under a FakeClock and never blocks the escalation handler. When the budget is spent the failure escalates. This completes the supervision strategy set — Stop, Escalate, Restart, Backoff — over the kernel's escalation seam. Coverage: cluster 96 percent. Additive; stdlib-only.