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.

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