[PAYLOAD-ORCH-01] Wire bounded direct-Engine payload building into Builder

[krisoshea-eth]

Why

After the payload-source boundary is reviewed, PAYLOAD-01 must connect authoritative source-BN inputs to one bounded direct-Engine build without importing Nico's complete bidding and reveal loop into a single PR.

Planning

• Lane: Builder/Engine API
• Effort: M
• Dependency-independent orchestration core in progress; final runtime wiring remains after PAYLOAD-SOURCE-01, EL-ARCH-01, and the PAYLOAD-01 input contract in BN-01
• Owner: Kris
• Reviewer: Lodestar maintainer
• Parent: PAYLOAD-01
• Architecture source: nflaig/builder at 99fd8fa9ad

Scope

• Add the accepted Engine URL, JWT, timeout, retry, and readiness configuration.
• Construct and inject the reviewed payload source during Builder initialization.
• Consume fork-correct payload attributes, safe and finalized execution hashes, proposer inputs required for the build, and the agreed post-Gloas custody_columns value.
• Implement the smallest correct single-source prepare and get-payload flow.
• Bound concurrent jobs, retries, deadlines, and stale work.
• Preserve abort and shutdown behavior.
• Return precise syncing, unsupported, timeout, no-payload, stale-input, and invalid-result outcomes.
• Expose the completed payload through the seam required by STORE-01.
• Add lifecycle, failure, cancellation, fork, and exact-width tests.

Out of scope

• Multi-EL best-value selection unless it remains a small extension after the single-source path is correct.
• Payload persistence.
• Bid construction or publication.
• Selection, reveal, and recovery.
• New Beacon API event contracts.

Done when

One fork-correct source-BN input causes one bounded direct-Engine payload build, and the complete result is available to STORE-01 through deterministic tests.

Evidence

• Fork-only stacked draft: feat(builder): coordinate bounded payload build jobs, targeting the PAYLOAD-SOURCE-01 branch so the review diff contains only the orchestration changes tracked by this issue
• EL ownership decision:
• Source-BN input contract:
• Orchestration and lifecycle tests: 12 focused tests on stacked commit df448c8d95f44c430cda38e30c1623750e17b27a; 51 Builder package tests passed
• Spike branch: krisoshea/lod-58-payload-orch-01-spike
• Spike boundary: caller-defined identity and retrieval time, bounded active jobs, duplicate sharing, phase-specific timeouts, cancellation, late-result suppression, and cleanup
• Still blocked: concrete Engine construction, shared/dedicated EL ownership, source-BN input production, and Builder/CLI wiring
• Reviewer:

[linear-code]

LOD-58