fix(validator): guard against uint64 underflow in Start() slot arithmetic

[ygd58]

Fixes #69.

phase0.Slot is uint64. In Start(), two subtractions can wrap around:

1. headSlot - finalizedL1Offset wraps if the beacon head is below slot 64. Added an early return with a descriptive error.
2. end - phase0.Slot(a.cfg.NumBlocks) wraps if the chain has fewer than NumBlocks slots. Clamped start to 0 in that case instead of underflowing.

The checkBlobs loop iterates slot := start to end, so a wrapped start value would cause the validator to hang iterating an astronomically large range. All existing tests pass.

[cb-heimdall]

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[ygd58]

hey @wbnns @danyalprout can you take a look? fixes a uint64 underflow in the validator Start() that would cause the blob checker to hang on a chain with fewer than 664 slots.

[wbnns]

@ygd58 Similarly to the other thread, I wouldn't be the right person to give guidance here. :)