Parallel phase execution in builders

[waleedkadous]

Feature Request

Add support for parallel phase execution in builders. Currently SPIR/ASPIR builders execute phases strictly sequentially, but plans often contain dependency diagrams showing phases that could run concurrently.

Problem

A typical plan might have:

Phase 1: Data layer
Phase 2: API routes (depends on Phase 1)
Phase 3: UI components (depends on Phase 1)
Phase 4: Integration (depends on Phase 2, Phase 3)

Today: Phase 1 → 2 → 3 → 4 (serial, ~40 min)
With parallelism: Phase 1 → (2 + 3 parallel) → 4 (~30 min)

Proposed Design

1. Plan format extension: Plans declare phase dependencies explicitly (not just ordering)
2. Porch orchestration: Porch reads dependency graph and launches independent phases concurrently
3. Multi-builder coordination: Each parallel phase gets its own builder in its own branch, merging results when all complete
4. Conflict resolution: When parallel phases touch overlapping files, automated merge with consultation fallback
5. Consultation gates: Phases that complete concurrently batch their consultations

Challenges

• Git merge conflicts between parallel branches
• Shared state — parallel phases may need to read each other's intermediate output
• Consultation ordering — how to consult on phases that landed simultaneously
• Progress tracking — dashboard needs to show parallel swimlanes
• Failure handling — if one parallel phase fails, what happens to siblings?

This is a large project

Likely needs its own spec with multiple sub-phases. May warrant an EXPERIMENT protocol first to validate the multi-builder merge approach before full implementation.