ethlambda-rpc tests fail in debug builds: u8 overflow in ptc.rs test state builder #786

Summary

60 ethlambda-rpc unit tests fail in debug builds with attempt to add with overflow. CI does not see it because tests run with the release-fast profile, where overflow checks are off.

Cause

crates/net/rpc/src/beacon/ptc.rs:574, in a test state builder:

for slot in 0..inner.block_roots.len() {
    inner.block_roots[slot] = H256::repeat_byte(slot as u8 + 1);
}

block_roots has more than 255 entries, so at slot == 255 the expression slot as u8 + 1 overflows (255u8 + 1). In release it wraps silently, which also means the "distinct roots per slot" intent breaks after slot 255 (roots repeat).

Reproduce

cargo test -p ethlambda-rpc --lib

(debug profile) → 60 failures at ptc.rs:574. Same result on main c3f5f56 and on #783.

Suggested fix

Use a root that stays distinct for every slot, e.g. H256::from_low_u64_be(slot as u64 + 1), or (slot as u8).wrapping_add(1) if distinctness beyond 255 doesn't matter. Consider running unit tests in debug (or with overflow-checks = true) in CI.