Fix uint64 underflow in slot arithmetic - #92

Summary

Two spots subtracted slot numbers without checking for underflow, both uint64.

ProposerDelay compared currSlot minus the block's slot against a threshold to skip old blocks. A block one slot ahead of the wallclock, which happens on ordinary clock skew, underflows that subtraction to a huge number and gets silently dropped from the metric instead of recorded.

Empty slot detection computed slot minus one to look up the previous block. At slot 0 that underflows to the max uint64 value and requests a nonsense block.

Test plan

• Extracted both comparisons into small pure functions (blockTooOldForProposerDelay, previousSlotStateID) and added table-driven tests for each in pkg/beacon/slot_arithmetic_test.go, covering past, current, future and slot-0 cases
• Confirmed both sets of tests fail against the old unguarded subtraction and pass against the fix
• go build ./..., go vet ./..., go test -race ./... all green