feat(engine): REST+SSZ engine API (execution-apis #793)

[barnabasbusa]

Summary

Switches the engine API from the #764 SSZ-over-JSON-RPC transport to the #793 resource-oriented REST API: fork-scoped endpoints under /engine/v2/{fork}/... with SSZ bodies over HTTP/2 (h2c via httpx), structured JSON for capabilities/identity, RFC-7807 problem responses, and the restricted PayloadStatus enum (no INVALID_BLOCK_HASH).

Legacy JSON-RPC stays as the transition-window fallback: if the EL doesn't expose /engine/v2/... (or doesn't advertise the URL fork via GET /engine/v2/capabilities), every call falls back to JSON-RPC for the connection's lifetime.

What changed

• engine/ssz_types.py — rewritten for #793: per-fork ExecutionPayloadEnvelope (params folded in, expectedBlobVersionedHashes removed), ForkchoiceUpdate (with Amsterdam custody_columns), BuiltPayload (new field order), blob-pool v1–v4 and historical-bodies containers, plus per-fork dispatch tables. New PayloadStatus (uint8 enum, Optional[String] validation_error) — serializes to exactly the 41 bytes in the spec's worked example.
• engine/client.py — httpx transport (h2c prior-knowledge for v2, HTTP/1.1 for the JSON-RPC fallback). GET /engine/v2/capabilities probe, GET /engine/v2/identity + X-Engine-Client-Version header, CL→EL fork mapping (gloas→amsterdam, …). newPayload/forkchoiceUpdated/getPayload/getBlobs/bodies route via v2 when the fork is advertised, else JSON-RPC.
• node.py — best-effort blob backfill from the EL pool (engine getBlobs) for gossiped blocks carrying commitments; reconstructs Deneb/Electra sidecars, fetches PeerDAS cell proofs (column reconstruction left to the existing data-column path). Runs in the background, never blocks import.
• pyproject.toml — add httpx[http2]; cli.py — update --engine-force-json help.

Notes / deliberate scoping

• target_gas_limit kept in the Amsterdam PayloadAttributes. #793's container sketch omits it, but geth's post-Amsterdam build path requires it (the sketch self-describes its types as placeholders).
• getPayloadBodies implemented but not wired into the node — consensoor syncs state over HTTP and the EL self-syncs, so there's no consumer yet. Client methods remain available + tested.
• Blob backfill uses a placeholder inclusion proof, matching the store's existing proposer-side blob handling (consensoor has no KZG-verify / merkle-proof machinery yet).
• #793 is a draft and no EL ships it yet, so against current devnets the v2 probe 404s and everything falls back to JSON-RPC (the spec's intended transition behavior). The real h2c wire path is unverified against a live v2-capable EL.

Test plan

• All #793 SSZ containers round-trip; PayloadStatus == 41-byte spec example
• Dispatcher routing (paris/cancun/amsterdam) builds correct paths + envelopes via mocked transport
• Capabilities probe: v2-enabled, fallback-to-JSON-RPC, --engine-force-json skip; identity v2
• get_blobs fork→revision selection (amsterdam→v4/v3, osaka→v3, cancun→v1, none→None)
• Node blob backfill: v1 reconstruction, partial backfill, cell-proof skip, EL-can't-serve no-op, already-have short-circuit
• End-to-end against a live /engine/v2-capable EL (none available yet)