dutytracer: unsigned underflow in Collector.evict() threshold at chain start

[ovidiu-ssv-labs]

Summary

Collector.evict() computes its eviction threshold with an unguarded unsigned subtraction:

https://github.com/ssvlabs/ssv/blob/main/operator/dutytracer/collector.go#L139-L147

const slotTTL = 4

func (c *Collector) evict(currentSlot phase0.Slot) {
	start := time.Now()
	threshold := currentSlot - slotTTL   // <-- underflows when currentSlot < slotTTL
	...
	c.lastEvictedSlot.Store(uint64(threshold))
	...
}

phase0.Slot is an unsigned integer. When currentSlot < slotTTL (i.e. the first few slots after genesis), currentSlot - slotTTL wraps around to a near-MaxUint64 value. That wrapped value is then Stored into lastEvictedSlot and passed as the threshold to dumpCommitteeToDBPeriodically / dumpValidatorToDBPeriodically / dumpLinkToDBPeriodically.

Impact

Low in practice — this only triggers in the first ~4 slots of a network's life, which no production exporter will realistically hit. Flagging it for correctness/consistency rather than as an active bug.

Context

This came out of reviewing #2895. That PR adds an analogous retention path (retentionState) that does carefully guard the same shape with a currentSlot <= retainSlots check before subtracting. evict() is the pre-existing sibling one function over that lacks the equivalent guard. Out of scope for #2895, so tracking separately here.

Suggested fix

Add the same underflow guard evict()'s sibling uses — early-return (or clamp the threshold to 0) when currentSlot <= slotTTL — so the two paths are consistent.