feat: PLA-1058 bound independent test case concurrency

[jeffatgametime]

The following code makes an impact.

Purpose of Change

Provide bounded opt-in concurrency for independent top-level hosted test cases without parallelizing ordered steps inside a case or losing cleanup ownership.

Ticket Link

PLA-1058

Change Overview

• Add explicit ConcurrentTest registration and copied canonical ListCases identities.
• Introduce one reusable CaseExecutor per hosted worker, shared across packages/runs. Ordinary cases and package hooks use exclusive admission.
• Hold admission through BeforeTest, body, sequential children, cleanup and AfterTest; propagate caller cancellation and drain admitted work before returning.
• Retain canonical results independent of completion order and expose separate QueueDur.
• Make native AfterPackage teardown wait for parallel children and their cleanup; preserve package-hook error diagnostics.

Stack dependency: #36 provides explicit skip/lifecycle semantics. This PR targets report-skipped-tests only while #36 is open. Do not merge this branch into that feature branch; after #36 merges, retarget to master, verify the resulting diff and rerun required checks/review.

Affected Functionality

Generic testy hosted execution and native teardown. Nil executor preserves serial-in-package behavior; size one serializes cases sharing that executor. ConcurrentTest is an explicit independence declaration, not blanket activation of legacy nested Parallel calls.

No api-tests cases are opted in by this library PR. This is a process-local bound, not a fleet-wide admission service or an exactly-once side-effect guarantee. Non-cooperative callbacks cannot be preempted.

Testing

• Primary and independent review: go test -race ./... -count=3 -timeout=90s pass.
• go vet ./... and git diff --check pass.
• Matching Go 1.21.13 + golangci-lint 1.54.2 run clean.
• Contract coverage: shared cross-package bound, permit ownership through cleanup/hooks, live exclusive case ordering, canceled queued work, partial exclusive cancellation rollback, cancellation/drain, panic/fatal/skip isolation, ordered child steps, nil/cap-one fallback, native teardown, registry snapshots and package-hook error diagnostics.
• Independent public-API diagnostic reproduction now retains BeforePackage Errorf in the failed package result.

Observability

TestResult.QueueDur measures admission delay separately from admitted lifecycle Dur. Canonical names/results and errors remain visible. Runtime peak load, queue/phase signals and exact consumer identity must be validated in the api-tests adoption workstream; no hosted speedup is claimed here.

Rollout

Merge #36 first, retarget and verify this PR on master, then normal required review/check gates. No tag or deployment from this PR-management task. api-tests must consume the approved immutable library identity separately. Root coordinates staged worker drain and maximum replica/surge/dependency budget before enabling overlap; retain serial rollback. Hosted consumer/load acceptance remains pending on PLA-1058.