# PR-013 - PBS Unresolved Import Diagnostics

## Briefing

Import resolution currently ignores missing target modules in frontend linking validation. This PR enforces deterministic diagnostics for unresolved module imports and unresolved imported symbols.

## Motivation

Silent import misses break the diagnostics contract and defer user-actionable errors.

## Target

- Module import validation in `PbsModuleVisibilityValidator`.
- Linking diagnostic coverage for import failures.

## Scope

- Emit errors for missing target module references.
- Keep existing public-symbol checks.
- Preserve deterministic, source-first attribution.

## Method

- Extend import validation to detect absent target modules.
- Report diagnostics on the importing site.
- Keep related-span behavior where applicable.

## Acceptance Criteria

- Importing a non-existent module always emits deterministic linking/import diagnostic.
- Importing a non-public symbol still emits existing diagnostic.
- Diagnostic attribution points to import site.
- No silent pass for unresolved imports remains.

## Tests

- Add tests for missing module import.
- Add tests for missing symbol import in existing module.
- Verify phase and diagnostic code stability.

## Non-Goals

- Full manifest/dependency resolver redesign.
- Loader/runtime capability checks.