feat: add test-scoped cleanup and context

[jeffatgametime]

The following code makes an impact.

Purpose of Change

Make test-owned resource cleanup safe and reusable without introducing another test framework. First dependency in the approved api-tests authoring stack.

Ticket Link

N/A — approved acceptance-authoring architecture work.

Change Overview

• Add optional testy.Cleanup and testy.Context capabilities without expanding TestingT or breaking existing custom implementations.
• Run LIFO cleanup on return, fatal assertion, panic, and nested subtests; cancel the test context before cleanup and continue earlier callbacks after cleanup failures.
• Keep native/hosted cleanup before AfterTest; document legacy hooks, custom-runner requirements, parallel behavior, and process-termination limits.

Affected Functionality

Native RunAsTest, hosted package execution, and callers adopting lifecycle helpers. Legacy Before/After hooks cannot acquire lifecycle-owned resources. No Temporal dependency or result-schema change.

Testing

• go test -race ./... passed.
• Both runners tested for normal/fatal/panic bodies, fatal/panic cleanup, nested registration, LIFO order and cancellation.
• Actual RunAsTest/runPackage hook-order integration; native parallel child lifetime; parent cancellation and deadline inheritance.
• Independent local review found an AfterTest ordering mismatch; fixed and re-reviewed.
• git diff --check passed.

Observability

Cleanup errors contribute to existing test failure results and diagnostics rather than disappearing as successful tests.

Rollout

Review/merge this library PR before adopting/releasing the capability. The api-tests companion will pin this immutable branch commit during review and document the release dependency. User requested PR creation only; no merge or deployment is performed.

[jeffatgametime]

Final verification at 54d1dc03a8a9535cbb22846b8bac3e6b1889a330: Go checks and lint are green. Local race/lifecycle tests passed, including failure and native/hosted hook parity. The api-tests companion pins this exact immutable commit and passed the Temporal local-stack smoke plus latest staging acceptance on both foundation and migration PRs: https://github.com/gametimesf/api-tests/pull/212 and https://github.com/gametimesf/api-tests/pull/213 . No outstanding findings or review threads. Ready for required human review; GitHub still reports REVIEW_REQUIRED. Nothing merged or released.