# PR-018 - PBS Result Flow (ok/err/handle) Rule Enforcement

## Briefing

Result-flow rules are only partially enforced. This PR enforces allowed positions for `ok/err`, error-label validation, and `handle` arm terminal form requirements.

## Motivation

Result-flow constructs are normative control-flow surfaces and must not behave as unconstrained expressions.

## Target

- `ok(...)` and `err(...)` validation in return-flow contexts.
- `handle` semantic validation, including terminal form of block arms.

## Scope

- Restrict `ok/err` to allowed result-flow positions.
- Validate `err(E.case)` against declared error type.
- In `handle` block arms, enforce terminal `ok(payload)` or `err(E2.case)`.

## Method

- Add expression-use/context flags for result-flow forms.
- Add dedicated diagnostics for invalid placement and invalid error labels.
- Add terminal-check pass for handle block arms.

## Acceptance Criteria

- `ok/err` outside allowed positions are rejected deterministically.
- `return ok/err` validates declared `result<E>` contract.
- `handle` block arms violating terminal rules are rejected deterministically.
- Existing propagate (`!`) and handle exhaustiveness checks remain stable.

## Tests

- Add positive and negative fixtures for `return ok/err`.
- Add invalid-position tests for `ok/err` as ordinary expression.
- Add `handle` arm terminal-form tests.

## Non-Goals

- New result syntax.
- Runtime trap policy changes.