prometeu-studio/docs/packer/pull-requests/PR-08-assets-pa-and-companion-artifact-emission.md

PR-08 assets.pa and Companion Artifact Emission

Domain Owner: docs/packer

Briefing

Implement writer-side artifact emission on top of the deterministic build planner.

This PR turns validated and planned build state into the published packer outputs.

Objective

Deliver build, assets.pa, and the baseline companion artifacts with deterministic ordering and stable writer-side contracts.

Dependencies

• ./PR-07-build-plan-determinism-and-cache-key-foundation.md
• ../specs/1. Domain and Artifact Boundary Specification.md
• ../specs/4. Build Artifacts and Deterministic Packing Specification.md

Scope

• implement the build service
• emit assets.pa
• emit:
  • build/asset_table.json
  • build/preload.json
  • build/asset_table_metadata.json
• implement canonical header serialization and file writing
• implement payload-region offset materialization from the build plan
• expose build summaries and applied-artifact metadata for Studio and CI

Non-Goals

• no cartridge assembly
• no shipper coupling
• no speculative layout optimization beyond deterministic packing

Execution Method

1. Convert the build plan into emitted artifact bytes and companion files.
2. Preserve header authority and 1:1 mirror rules for companion JSON outputs.
3. Add golden fixtures for emitted headers and companion artifacts.
4. Keep runtime-facing semantics bounded to writer-side production rules already specified.

Acceptance Criteria

• build produces assets.pa and companion artifacts deterministically
• companion JSON outputs mirror the runtime header where required
• emitted ordering and offsets follow the build plan contract
• repeated equivalent builds do not drift

Validation

• golden tests for assets.pa envelope and header bytes
• golden tests for companion JSON mirrors
• repeated-build determinism tests

Affected Artifacts

• prometeu-packer/**
• build output fixtures
• CI artifact assertions