feat(builder): assemble stateless payload envelopes

[krisoshea-eth]

Motivation

A selected local bid must reveal only the exact payload material that was retained for that bid. Envelope construction should fail closed if the retained payload belongs to a different slot, parent execution block, or execution block hash.

Description

Adds a pure createExecutionPayloadEnvelopeMaterial boundary that:

• accepts the selecting Beacon block root, local Builder index, selected bid identity, and retained payload material carrying its original parent Beacon block root;
• verifies slot, parent execution block hash, parent Beacon block root, and execution block hash;
• constructs the stateless ExecutionPayloadEnvelope without mutating retained material;
• preserves execution requests, blobs, KZG proofs, Builder index, Beacon block root, and parent Beacon block root;
• supports both Gloas and Heze retained payloads;
• reports structured mismatch errors.

Signing, API publication, reveal cutoffs, retries, store eviction, metrics, and Builder wiring remain outside this draft.

This draft is stacked on #9958 for the BuiltPayload contract. The envelope slice itself is one commit and two files.

Tracking: LOD-66

Tests

• 6 focused envelope-assembly tests
• Builder type-check
• Biome check
• Builder build and import check
• git diff --check

Review plan

Treat #9980, #9981, and #9982 as one logical selection and reveal path. Keep this draft isolated during development, but combine the services around their first runtime consumer if maintainers prefer fewer service-only PRs.

This PR was written primarily with Codex. I reviewed the implementation against Lodestar types, the project reveal requirements, and Nico's Builder proof of concept, and validated it locally.