Add state diff serialization (#15250)

* Add serialization code for state diffs

Adds serialization code for state diffs.
Adds code to create and apply state diffs
Adds fuzz tests and benchmarks for serialization/deserialization

Co-authored-by: Claude <noreply@anthropic.com>

* Add Fulu support

* Review #1

* gazelle

* Fix some fuzzers

* Failing cases from the fuzzers in consensus-types/hdiff

* Fix more fuzz tests

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* add comparison tests

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Use ConvertToElectra in UpgradeToElectra

* Add comments on constants

* Fix readEth1Data

* remove colons from error messages

* Add design doc

* Apply suggestions from code review

Bast

Co-authored-by: Bastin <43618253+Inspector-Butters@users.noreply.github.com>

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Preston Van Loon <preston@pvl.dev>
Co-authored-by: Bastin <43618253+Inspector-Butters@users.noreply.github.com>

beacon-chain/state/state-native/state_trie.go

@@ -650,6 +650,11 @@ func InitializeFromProtoUnsafeFulu(st *ethpb.BeaconStateFulu) (state.BeaconState
 	for i, v := range st.ProposerLookahead {
 		proposerLookahead[i] = primitives.ValidatorIndex(v)
 	}
+	// Proposer lookahead must be exactly 2 * SLOTS_PER_EPOCH in length. We fill in with zeroes instead of erroring out here
+	for i := len(proposerLookahead); i < 2*fieldparams.SlotsPerEpoch; i++ {
+		proposerLookahead = append(proposerLookahead, 0)
+	}
+
 	fieldCount := params.BeaconConfig().BeaconStateFuluFieldCount
 	b := &BeaconState{
 		version:                           version.Fulu,