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.

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