### Phase 03 — Frontend API Boundary (Canon Contract)

This document codifies the FE/BE boundary invariants for Phase 03.

- BE is the source of truth. The `frontend-api` crate defines canonical models that all Frontends must produce.
- No string protocols across layers. Strings are only for display/debug. No hidden prefixes like `svc:` or `@dep:`.
- No FE implementation imports from other FE implementations.
- No BE imports PBS modules (hard boundary). The Backend consumes only canonical data structures from `frontend-api`.
- Overload resolution is signature-based. Arity alone is not sufficient; use canonical signatures/keys.

Implementation notes (PBS):

- The PBS adapter must not synthesize ownership or module info from string prefixes. All owner/module data should come from canonical types.
- Export/import surfaces are expressed exclusively via `frontend-api` types (e.g., `ItemName`, `ProjectAlias`, `ModulePath`, `ImportRef`, `ExportItem`).

Enforcement:

- A test in `prometeu-build-pipeline` scans `src/backend/**` to ensure no references to `frontends::pbs` are introduced.
- Code review should reject any PRs that reintroduce prefix-based heuristics or FE-to-FE coupling.