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.

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