[proto] add evaluation completed event contract

[haasonsaas]

Summary

• add a typed events.v1.EvaluationCompleted payload for canonical evaluation.completed bus events
• add a canonical CloudEvent fixture for the fermata -> pipeline technical capability seam
• expose fixture/helper/package support so downstream repos can pin consumer tests to the shared contract

Testing

• buf lint
• buf generate
• buf breaking --against ".git#branch=main"
• go test ./...
• npm run check:package

[cursor]

PR Summary

Medium Risk
Adds a new cross-service events.v1.EvaluationCompleted schema and wires it into helpers/fixtures and the published TS package surface, which could impact downstream consumers that rely on event contract stability. Changes are additive but touch shared event envelope tooling and contract tests.

Overview
Introduces a new typed bus payload, events/v1.EvaluationCompleted, for canonical evaluation.completed CloudEvents, with generated Go/TS code committed.

Adds a canonical protojson CloudEvent fixture for the Fermata → Pipeline seam and validates it via new contract tests, plus exposes it through contractfixtures with a new LoadEvaluationCompletedFixture helper.

Extends eventhelpers with UnpackEvaluationCompleted, updates documentation to reference the new helper/fixture, and updates package.json exports and the check:package script to include the new TS module.

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