FE-829: Build-architect for spec-derived cook plans

[kostandinang]

Stack Context

Stacked on FE-827 (#182). Part of the cook/orchestrator track: FE-800 (spec→cook-plan emitter) → FE-826 (spec-derived mode) → FE-827 (policy-selected slice layout + greenfield promotion) → FE-829.

This frontier evolves brunch plan from a sequencer into a build-architect that emits fixture-quality, cook-executable plan.yaml — closing the gap to the hand-authored reference fixtures (parallel-utils, layered-todo, resilient-pipeline), which already execute correctly in the orchestrator.

Why?

FE-800 makes brunch plan produce a faithful but un-buildable projection of the spec's requirement graph: plausible-looking depends_on that are conceptual references rather than build order, no decomposition, no join slice, no epic integration seam — so multi-slice epics produce "green checks, no assembled artifact" (the FE-800 integration-blind follow-on).

What (this PR — planning only)

Planning commit that registers the frontier and records the design in canonical docs. No code yet — draft for visibility while slice 1 is built.

• SPEC: A100-K (contract-completeness assumption), D167-K (emitter guarantees cook-executability via a producer-agnostic PlanContract + deterministic repair), I129-K (every emitted plan satisfies the executability contract), Future Direction §Cook plan generation (build-architect arc + the deferred D160-K amendment).
• PLAN: plan-build-architect frontier — Sequencing (Active) + Frontier Definition with the four-slice breakdown.

Slices

1. Executability contract + deterministic repair (building now) — self-contained PlanContract.check (acyclic deps over existing ids; ≥1 verification target/slice; slice∈epic; requirement coverage; multi-slice epic carries an integration-test seam) + a repair loop that auto-fixes the mechanical class (cycle-break, mint target, synthesize epic seam) and surfaces the design class as typed warnings. No LLM, no D160-K change.
2. Build-architect LLM stage — criteria + edges fed in, schema-constrained, reference fixtures inlined as few-shot exemplars.
3. File-ownership / join synthesis — requires the D160-K amendment + a Slice.writes schema field.
4. Eval harness — score generated plans on contract-conformance + structural similarity to the reference fixtures.

🤖 Generated with Claude Code

[kostandinang]

Superseded by #185. The branch name ka/fe-829-plan-build-architect collided with merged PR #183's fallback headRefName, which shadowed this PR out of the Graphite stack. Recreated on ka/fe-829-build-architect via gt submit.