PR-07 Build Plan, Determinism, and Cache-Key Foundation

Domain Owner: docs/packer

Briefing

Implement the deterministic planning layer that normalizes validated assets into a stable build plan before artifact emission.

This PR is the boundary between workspace semantics and runtime-facing artifact production.

Objective

Deliver a deterministic build planner, canonical header model, preload derivation, and cache-key foundation that later emission and incremental work can trust.

Dependencies

Scope

validate build-eligible managed assets
derive deterministic asset ordering by increasing asset_id
build canonical header-side models for:
- asset_table
- preload
define payload planning slots and offset planning inputs
compute cache keys from validated effective inputs
emit deterministic in-memory build plans before writing files

Non-Goals

no final assets.pa file emission yet
no watch mode yet
no format-family semantic expansion beyond what current specs already define

Execution Method

Normalize validated managed assets into a stable build plan representation.
Implement canonical JSON serialization rules for header mirrors and planner assertions.
Compute cache keys only from validated effective inputs.
Add deterministic tests proving equivalent workspaces produce equivalent plans.

Acceptance Criteria

equivalent inputs produce equivalent build plans
asset ordering is independent from filesystem traversal order
preload derivation matches declaration state deterministically
cache-key inputs are explicit and reproducible

Validation

determinism tests with reordered filesystem fixtures
canonical-header serialization tests
cache-key stability tests

Affected Artifacts

prometeu-packer/**
build planning fixtures
future dependency for build service and event lane

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