`engine_forkchoiceUpdatedV1`/`V2`/`V3` allow beginning payload build process if `forkchoiceState.headBlockHash` references a `VALID` ancestor of the head of canonical chain

[tersec]

https://github.com/ethereum/execution-apis/blob/v1.0.0-beta.4/src/engine/paris.md#specification-1 notes:

2. Client software MAY skip an update of the forkchoice state and MUST NOT begin a payload build process if forkchoiceState.headBlockHash references a VALID ancestor of the head of canonical chain, i.e. the ancestor passed payload validation process and deemed VALID. In the case of such an event, client software MUST return {payloadStatus: {status: VALID, latestValidHash: forkchoiceState.headBlockHash, validationError: null}, payloadId: null}.

But

nimbus-eth1/nimbus/beacon/api_handler/api_forkchoice.nim

Lines 130 to 136 in bda760f

 # If the head block is already in our canonical chain, the beacon client is 
 # probably resyncing. Ignore the update. 
 var canonHash: common.Hash256 
 if db.getBlockHash(header.blockNumber, canonHash) and canonHash == blockHash: 
   # TODO should this be possible? 
   # If we allow these types of reorgs, we will do lots and lots of reorgs during sync 
   notice "Reorg to previous block", blockHash 

combined with

nimbus-eth1/nimbus/beacon/api_handler/api_forkchoice.nim

Lines 188 to 207 in bda760f

 # If payload generation was requested, create a new block to be potentially 
 # sealed by the beacon client. The payload will be requested later, and we 
 # might replace it arbitrarilly many times in between. 
 if attrsOpt.isSome: 
   let attrs = attrsOpt.get() 
   validateVersion(attrs, com, apiVersion) 
  
   let bundle = ben.generatePayload(attrs).valueOr: 
     error "Failed to create sealing payload", err = error 
     raise invalidAttr(error) 
  
   let id = computePayloadId(blockHash, attrs) 
   ben.put(id, ben.blockValue, bundle.executionPayload, bundle.blobsBundle) 
  
   info "Created payload for sealing", 
     id = id.toHex, 
     hash = bundle.executionPayload.blockHash.short, 
     number = bundle.executionPayload.blockNumber 
  
   return validFCU(some(id), blockHash) 

allow exactly this.