Add typed assertion-kind constants

[korya]

Problem

Library consumers must compare assertion families through raw strings, making structured result handling typo-prone and undiscoverable.

FailureCode and EvaluationErrorCode already provide typed discriminators, but Assertion.Kind, Outcome.Kind, Failure.Kind, and EvaluationError.Kind expose plain strings. This inconsistency would become costly once v0.4.0 publishes the package contract.

Solution

Introduce AssertionKind and exported constants across the public result model without changing their string values.

Before:

if outcome.Kind == "jq" {
    // handle jq
}

After:

if outcome.Kind == ha.KindJQ {
    // handle jq
}

Built-in assertions now return the corresponding constant. The type remains an open string type, so external packages can define custom assertion families rather than being restricted to the built-in set. CLI output and machine-visible values remain byte-for-byte compatible.

No visual change: this changes a Go API type without changing CLI rendering.

Other Changes

• Add an external-package compile-time assertion proving custom implementations remain supported.
• Record the typed discriminator in the unreleased changelog.

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