engine: fix stale references in the REST-SSZ docs

[LukaszRozmej]

Editorial follow-up to ethereum#793, which merged with a few doc-level items from the implementer feedback still open. Both came out of implementing the REST+SSZ surface in Nethermind.

1. The worked byte example contradicts the status enum

refactor-ssz.md pins VALID = 0 and its Example A correctly shows status: 0x00, but refactor.md § "Example: submit a payload" describes the same 41-byte response as status (1 byte = 0x01, VALID). Since the point of that section is a byte-exact example, it is the version most likely to be copied into a test vector.

2. Stale v2 / fork-in-URL references

d39e9a27 moved the fork from the URL into Eth-Execution-Version and renumbered the base path to /engine/v1, but a few places still describe the earlier shape:

• refactor-ssz.md was titled "Engine API v2 -- SSZ Container Sketches" and referred to "the Engine API v2 spec" / "the v2 API".
• refactor.md's goals section still said the new API "puts the fork in the URL (/engine/v1/...)".
• refactor-ssz.md linked #get-forkpayloadspayloadid, which no longer resolves — the heading is now GET /payloads/{payloadId}, and the doc's own table of contents already uses #get-payloadspayloadid.

This is the one that cost us real time: reading the goals section and the PR description table, we kept /engine/v2/{fork}/... while picking up the header move, so a CL following the draft got a 404 on every REST URL — and per the transition-window section a 404 reads as "this EL has no REST surface, fall back to JSON-RPC", making the divergence silent rather than loud.

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No normative changes here. Three other items from that feedback are deliberately left out, since each needs a decision rather than an edit:

• payload_id TTL vs the polling model — "valid until … the payload was retrieved" invalidates the token on the first GET, which makes the polling described a few lines above impossible. Review on engine: add Rest-SSZ spec ethereum/execution-apis#793 pointed at the polling reading ("a token ttl should be longer than a single get", engine: add Rest-SSZ spec ethereum/execution-apis#793 (comment)); a wording fix carrying that intent is ready to file separately.
• 413 request-too-large vs 400 ssz-decode-error for the SSZ count limits (bodies.max_count, blobs.max_versioned_hashes), which are also the container MAX_* bounds — filed separately.
• MAX_BAL_BYTES (2**30) being 16x MAX_REQUEST_BODY_SIZE (2**26), already listed as an open sketch question.