add Explorer recipe lifecycle and publication orchestration

[matthewpeterkort]

Loom Explorer Builder stack

Layer 2 of 6. Merge strictly in order: #17 → #18 → #19 → #20 → #21 → #22.

Previous: #17
Next: #19

What changed

• freezes the Explorer builder v1 contracts
• implements the project recipe draft, revision, validation, preview, and activation lifecycle
• adds project recipe publication orchestration
• hardens recipe lifecycle and publication boundaries

Why

This layer establishes the lifecycle contract and server behavior needed by a frontend-driven recipe builder.

Impact

Loom can support recipe authoring and publication without requiring the frontend to construct backend state implicitly.

Validation

The complete stack tip (b281499c) passes go test ./.... This layer also runs independent GitHub CI, generated-code checks, and a server image build.

[matthewpeterkort]

Archiving this superseded stack layer. Current stable baseline is PR #17 at 6c49754. The branch is preserved for historical reference; future Explorer work should be rebased or selectively ported onto current #17.