perf(blockchain): import blocks and envelopes before the engine answers

CLAUDE.md

@@ -83,6 +83,20 @@ Fork choice head update
 (Store buffer fields are `new_payloads`/`known_payloads`; the accessors are named
 `extract_latest_new_attestations`/`extract_latest_known_attestations`.)
 
+### Asynchronous `engine_newPayload` (beacon)
+`crates/blockchain/src/beacon_new_payload.rs`. On the running actor a pre-gloas
+block (optimistic candidate) imports as optimistic and a gloas envelope applies
+as `NOT_VALIDATED` before the EL answers; `engine_newPayload` goes out first from
+a tokio task that reports back with a `NewPayloadAnswered` message, applied as
+`optimistic-sync.md` applies a verdict. Sent parent-before-child, at most
+`MAX_NEW_PAYLOADS_IN_FLIGHT` in flight; `forkchoiceUpdated` names the newest
+payload on the head's chain whose request is not queued or in flight (so catch-up
+still feeds the EL a head); at most `MAX_PAYLOADS_AWAITING_VERDICT` wait (past
+that the import is held). No answer:
+kept optimistic, re-asked once a slot (a documented deviation). Without a
+mailbox (offline replay) and for non-candidates the import still waits inline.
+The actor records its own `ActorRef` in a `#[started]` hook for this.
+
 ### State Transition Phases
 1. **process_slots()**: Advance through empty slots, update historical roots
 2. **process_block()**: Validate header → process attestations → update justifications/finality
@@ -819,10 +833,13 @@ transitions are in `ethlambda-types`, per the section above. Nothing above
   the duties. `attestation_data.index` at a gloas slot comes from
   `Store::head_payload_status`.
   With `--execution-endpoint`, an envelope that passes the consensus checks is
-  judged by `engine_newPayloadV5` (INVALID with a null `latestValidHash` is a
-  hash mismatch that refuses only that envelope; INVALID with one condemns the
-  payload via the execution-chain walk; an engine error keeps the envelope and
-  retries once a slot, probing with the oldest), and a gloas head sends
+  applied at once (payload `NOT_VALIDATED`) and judged by `engine_newPayloadV5`
+  without waiting, like a pre-gloas block (see `beacon_new_payload` below):
+  INVALID with a null `latestValidHash` is a hash mismatch (the only answer
+  Engine API V2+ gives one) that takes the envelope back out and refuses only
+  that copy; INVALID with one condemns the payload via the execution-chain
+  walk; no answer keeps it applied and re-asks once a slot. Blocks built on a
+  FULL payload wait for its answer, not just its envelope. A gloas head sends
   `engine_forkchoiceUpdatedV4` with the custody columns (head hash by the head
   node's FULL or EMPTY status, finalized and safe from the checkpoint blocks'
   `bid.parent_block_hash`, safe being a fallback for fast confirmation). Building