# DESIGN — F-001 Scaffold modular monolith skeleton

## Affected modules
- New: `project/` application skeleton (no business modules yet, only exemplar `health` module).

## Modules touched
- `src/modules/health` (exemplar module proving the layout and boundary rule)
- `src/shared` (error envelope helper)
- `src/infrastructure/http` (Fastify server bootstrap)
- `src/app` (composition root)

## Modules NOT touched
- Everything else. No business modules exist yet. No harness files outside `work/artifacts/` and `specs/`.

## New interfaces
- `health` module public API: `registerHealthRoutes(app: FastifyInstance): Promise<void>` exported only from `src/modules/health/index.ts`.
- Boundary checker script: `node scripts/check-module-boundaries.mjs src` → exit 0 ok / exit 1 violation.

## API changes
- Adds `GET /health` → `200 {"status":"ok"}`.

## Database changes
- None.

## Events
- None.

## External integrations
- None.

## Cache changes
- None.

## Security considerations
- Fastify default JSON error handler replaced with an envelope that never leaks stack traces.
- No dependencies beyond Fastify + toolchain.

## Layout

```text
project/
├── package.json
├── tsconfig.json
├── eslint.config.mjs
├── vitest.config.ts
├── .gitignore
├── scripts/
│   └── check-module-boundaries.mjs
└── src/
    ├── app/
    │   └── build-app.ts            # composition root: wires modules
    ├── infrastructure/
    │   └── http/
    │       └── server.ts           # listen entrypoint
    ├── modules/
    │   └── health/
    │       ├── index.ts            # public API
    │       ├── api/
    │       │   └── health.routes.ts
    │       └── tests/
    │           └── health.test.ts
    └── shared/
        └── errors.ts
```

## Boundary rules enforced by the checker
1. Files inside `src/modules/<mod>/` may only import: own subtree (relative), `src/shared/...` (relative or alias-free path), Node builtins, and npm packages.
2. Any relative import escaping `src/modules/<mod>/` toward another module or toward `src/app`/`src/infrastructure` is a violation.
3. Files outside modules (`src/app`, `src/infrastructure`) may import a module only via its `index.ts` (direct deep import = violation).

## Toolchain
- TypeScript strict, Fastify 5, Vitest, ESLint (flat config) + Prettier, tsc build to `dist/`.

## Migration strategy
- None (greenfield).

## Rollback strategy
- Delete `project/` content added by this ticket; no other system depends on it yet.