# PR-020 - PBS Lowering Admission Gates

## Briefing

Lowering currently proceeds even when syntax/static/linking diagnostics contain errors. This PR introduces explicit admission gates so invalid programs are rejected before IR lowering.

## Motivation

Lowering must not convert required rejections into accepted lowered artifacts.

## Target

- `PbsFrontendCompiler` and `PBSFrontendPhaseService` lowering flow.
- Frontend-to-IR admission policy.

## Scope

- Block IR emission for source units with fatal diagnostics.
- Preserve deterministic diagnostics already emitted by previous phases.
- Keep successful paths unchanged.

## Method

- Introduce phase gate checks before `lowerFunctions` / file merge.
- Ensure failed units are excluded from emitted IR.
- Keep build issue adaptation behavior stable.

## Acceptance Criteria

- Files/modules with syntax/static/linking errors are not lowered.
- No invalid callable appears in IR output after failed admission.
- Valid files continue lowering unchanged.

## Tests

- Add tests proving failed parse/semantic/linking input does not emit IR functions.
- Add tests proving clean input still emits expected IR.
- Run frontend phase service and compiler suites.

## Non-Goals

- Redefining diagnostics severity policy.
- Introducing partial lowering recovery strategy.