# PR-019 - PBS Non-Unit Fallthrough Diagnostics

## Briefing

The frontend does not currently reject fallthrough of plain non-unit functions and `result` functions at end-of-body paths. This PR adds control-flow completion checks.

## Motivation

Return-surface obligations are normative and must be statically enforced.

## Target

- Callable body completion validation for function/method/ctor contexts.

## Scope

- Reject plain non-unit callable bodies that may complete without explicit return.
- Reject `result<E>` callable bodies that may complete without explicit return.
- Preserve existing unit and optional fallthrough behavior.

## Method

- Add path-completion analysis over statement/block graph.
- Integrate with existing flow body analyzer.
- Emit deterministic diagnostics with stable code/phase.

## Acceptance Criteria

- Possible fallthrough on plain non-unit callable is rejected.
- Possible fallthrough on `result<E>` callable is rejected.
- `void` and `optional` callable fallthrough behavior remains valid.

## Tests

- Add positive/negative control-flow completion fixtures.
- Cover nested `if`/`switch`/loop interactions.
- Run full flow semantics suite.

## Non-Goals

- Dataflow-based optimization.
- Runtime control-flow instrumentation.