Adds Invoke.Handler conformance case

[johnnyt]

Why

The Statifier.Invoke.Handler contract (ADR-0051 decision 4, docs/extending.md) binds downstream implementations only as prose: planning callbacks must be pure, perform/2 must be idempotent on invoke_id, cancel of an unknown invocation must not raise, {:error, _} from start/2 becomes error.execution, and exceptions are never rescued by the library. st-718j asks for a mechanical pin any handler implementation can run against itself, so every host handler gets contract coverage from day one and the contract stays pinned by tests rather than docs alone.

What

• Statifier.Testing.HandlerCase (new, ADR-0065): used alongside the host's own use ExUnit.Case with handler: and type: options, it generates seven conformance tests. Every check is also a public assertion function for suites that want them individually; fixtures (conformance_invoke/0, conformance_ctx/0, conformance_event/0) are overridable.
• Idempotency is judged against an implementor-declared observation point (observed_effects/1); a handler that routes work to perform/2 with none declared flunks naming the override to write (fail-not-skip, ADR-0053), never passes silently.
• The two library-half pins (error.execution surfacing via a minimal driving chart, exception propagation) run with case-internal probe handlers registered under the implementor's own type string, so they execute in every downstream suite.
• Docs: new "Testing your handler" section in docs/extending.md; ADR-0065 records the shape decisions (split-use over a second CaseTemplate, probe pins, observation-point discipline).
• Changelog fragment changelog.d/st-718j.md - a published testing module is public API.

Notes

• Independent of Adds caller_context per ADR-0063 #215 (caller_context); ADR-0065 records caller-context-aware conformance assertions as a follow-up once that lands, not a rider here.
• Every new literal test carries a verified sabotage note; the seven generated tests have no literal test blocks in the test file, so the two chart-driven library pins were sabotage-verified against Statifier.Session.Effects.plan_invoke/3 and the mutations are documented in a comment above the use site.
• Gate: full mix quality green (2,497 tests, 96.3% coverage) and mix quality --profile merge green including the ADR judge. Skips were not-applicable only (gettext).

Closes: st-718j