witness-retrieval: simplify encoding, drop error handling, de-link EIP-8025

- ExecutionWitnessV1 is now DATA representing SSZ-encoded ExecutionWitness
  bytes, following the same on-wire convention as BlobV1. Removed the
  `format` tag and the wrapper struct. Container schema deferred to a
  generic consensus-specs companion document.
- Removed `-38007 Witness unavailable` and its handling clauses; internal
  collection/serialisation failures fall through to standard JSON-RPC
  error semantics, consistent with the rest of the Engine API.
- Stripped all EIP-8025 references so the proposal stands alone. EIP-8025
  motivation can be supplied externally via the PR description.

Author: frisitano

src/engine/witness-retrieval.md

@@ -9,9 +9,8 @@ This specification is based on and extends [Engine API - Amsterdam](./amsterdam.
 > **Status:** Design proposal. This document is offered as an alternative to
 > introducing dedicated `*WithWitness` methods (cf. issue
 > [#741](https://github.com/ethereum/execution-apis/issues/741) and PR
-> [#773](https://github.com/ethereum/execution-apis/pull/773)). It is intended to
-> be evaluated by the EIP-8025 author team and Engine API maintainers prior to
-> merging.
+> [#773](https://github.com/ethereum/execution-apis/pull/773)). It is
+> intended to be evaluated by Engine API maintainers prior to merging.
 
 ## Table of contents
 
@@ -27,19 +26,17 @@ This specification is based on and extends [Engine API - Amsterdam](./amsterdam.
   - [engine_newPayloadV6](#engine_newpayloadv6)
   - [engine_getPayloadV7](#engine_getpayloadv7)
   - [engine_forkchoiceUpdatedV5](#engine_forkchoiceupdatedv5)
-- [Errors](#errors)
 - [Rationale](#rationale)
 - [Capabilities and feature gating](#capabilities-and-feature-gating)
 
 <!-- END doctoc generated TOC please keep comment here to allow auto update -->
 
 ## Motivation
 
-Stateless validation flows (e.g. EIP-8025 Optional Execution Proofs) require the
-consensus layer (CL) — or a prover acting on its behalf — to obtain the
-**execution witness** for a block: the set of state and bytecode accesses
-produced during execution. Two paths exist for the witness to leave the
-execution layer (EL):
+Stateless validation flows require the consensus layer (CL) — or a prover
+acting on its behalf — to obtain the **execution witness** for a block:
+the set of state and bytecode accesses produced during execution. Two
+paths exist for the witness to leave the execution layer (EL):
 
 1. **The CL receives an inbound payload over gossip.** The CL forwards it to the
    EL via `engine_newPayload*`, and wants the EL to attach the witness to the
@@ -64,19 +61,18 @@ added.
 
 ### ExecutionWitnessV1
 
-The execution witness collected by the EL during block execution. Its
-serialised form is a byte string; the binary encoding (SSZ container, RLP
-encoding, or other) is defined in the consensus-side companion specification
-and is out of scope for this document.
+`DATA` - Variable-length bytes representing an SSZ-encoded
+`ExecutionWitness` container. The container schema is defined by an
+accompanying consensus-specs document and is out of scope for this
+specification.
 
-The fields are encoded as follows:
-
-- `data`: `DATA` - Variable-length serialised execution witness.
-- `format`: `String` - Identifier of the witness encoding format. One of:
-  - `"ssz"` - SSZ-encoded `ExecutionWitness` container as defined in the
-    consensus-specs companion to EIP-8025.
-  - `"rlp"` - RLP-encoded witness format used by current pre-merge witness
-    tooling (e.g. Geth's `debug_executionWitness`).
+This follows the same on-wire convention used elsewhere in the Engine API
+for SSZ payloads (cf. the `blob` field in [`BlobV1`](./cancun.md)): a
+JSON-RPC `DATA` value whose underlying bytes are SSZ-encoded according to
+a consensus-specs definition. The encoding is fixed by this specification,
+so no format tag is required; on SSZ-REST transports (cf.
+[PR #764](https://github.com/ethereum/execution-apis/pull/764)) the same
+bytes are carried directly without hex wrapping.
 
 ### PayloadStatusV2
 
@@ -89,9 +85,10 @@ and appends a single optional field: `executionWitness`.
   block in the branch defined by payload and its ancestors.
 - `validationError`: `String|null` - A message providing additional details on
   the validation error if the payload is classified as `INVALID`.
-- `executionWitness`: `Object|null` - An [`ExecutionWitnessV1`](#executionwitnessv1)
-  object, present **only if** `requestWitness` was set to `true` in the
-  associated request **and** `status` is `VALID`. `null` in all other cases.
+- `executionWitness`: [`ExecutionWitnessV1`](#executionwitnessv1)`|null` -
+  SSZ-encoded execution witness. Present **only if** `requestWitness` was
+  set to `true` in the associated request **and** `status` is `VALID`.
+  `null` in all other cases.
 
 ### PayloadAttributesV5
 
@@ -154,10 +151,8 @@ additions:
    `result.executionWitness` to `null` and **MUST NOT** perform additional work
    to collect the witness on the critical path.
 
-4. Witness collection **MUST NOT** alter the validation outcome. A failure to
-   collect or serialise the witness while `requestWitness` is `true` and the
-   payload otherwise validates **MUST** return error code `-38007: Witness
-   unavailable` with the validation outcome unchanged on the EL.
+4. Witness collection **MUST NOT** affect the validation outcome reported
+   in `result.status`.
 
 5. Clients that have not advertised the `engine_newPayloadV6` capability via
    capabilities exchange **MUST** continue to be addressed via
@@ -185,9 +180,10 @@ with `payloadAttributes.requestWitness = true`.
   - `blobsBundle`: [`BlobsBundleV2`](./osaka.md#blobsbundlev2) - Bundle with data corresponding to blob transactions included into `executionPayload`
   - `shouldOverrideBuilder` : `BOOLEAN` - Suggestion from the execution layer to use this `executionPayload` instead of an externally provided one
   - `executionRequests`: `Array of DATA` - Execution layer triggered requests obtained from the `executionPayload` transaction execution.
-  - `executionWitness`: `Object|null` - An [`ExecutionWitnessV1`](#executionwitnessv1)
-    object, present **only if** the originating `engine_forkchoiceUpdatedV5`
-    call set `payloadAttributes.requestWitness = true`. `null` otherwise.
+  - `executionWitness`: [`ExecutionWitnessV1`](#executionwitnessv1)`|null` -
+    SSZ-encoded execution witness. Present **only if** the originating
+    `engine_forkchoiceUpdatedV5` call set
+    `payloadAttributes.requestWitness = true`. `null` otherwise.
 * error: code and message set in case an exception happens while getting the payload.
 
 #### Specification
@@ -205,12 +201,6 @@ additions:
    (or omitted), the EL **MUST** set `result.executionWitness` to `null` and
    **MUST NOT** perform additional work to collect the witness retroactively.
 
-3. If the EL was unable to collect or serialise the witness while
-   `requestWitness` was `true`, the EL **MUST** return error code `-38007:
-   Witness unavailable`. The CL **MAY** retry retrieval via
-   [`engine_newPayloadV6`](#engine_newpayloadv6) with `requestWitness = true`
-   on the produced payload.
-
 ### engine_forkchoiceUpdatedV5
 
 This method extends [`engine_forkchoiceUpdatedV4`](./amsterdam.md#engine_forkchoiceupdatedv4)
@@ -249,14 +239,6 @@ the following changes:
    **MUST NOT** perform additional work to collect the witness for this build
    job.
 
-## Errors
-
-This document introduces one new error code:
-
-| Code   | Message              | Meaning                                                                  |
-|--------|----------------------|--------------------------------------------------------------------------|
-| -38007 | Witness unavailable  | The EL was unable to collect or serialise the requested execution witness. |
-
 ## Rationale
 
 ### Why a flag, not a parallel method
@@ -333,6 +315,7 @@ prior capability confirmation from the EL. ELs that do not yet support
 witness retrieval continue to be addressed via the V5 / V4 / V6 predecessors
 unmodified.
 
-EIP-8025 activation is independent of this capability: an EL may advertise
-witness-retrieval support across all post-Amsterdam blocks regardless of
-EIP-8025 fork status, since the flag is opt-in per call.
+Witness-retrieval support is independent of any specific fork activation:
+once advertised, an EL may serve witnesses for any post-Amsterdam block
+the CL requests them for. Because the flag is opt-in per call, the EL
+incurs witness-collection cost only when a CL asks for it.