test: preserve late-failure teardown evidence

[seonghobae]

Partial #28. Stacked on current PR #144 exact head fea7bec6a3ef3f378884b717d36028d9103bde92. Current exact head is 5ee76849a1dc22f10ddcd291c8bac4cd612fa6a1.

Late-failure teardown TDD

• Test-only RED head f65d1ba45939605d577203e2bb1d1b3f183fbba1: CI 31680812328 / Rust contracts job 94385604530 failed at the new late-failure process-set teardown contract while production still returned only browser-root termination evidence.
• Production head 69b43c004c7c93419069bff472eac6b50d0ef97c moved sampled Chromium process-set shutdown verification before the failure return and propagated optional teardown evidence through the trial result.
• Head 77baa9581596dd2b195afb03182c5c730a704685 removed an irrelevant dict-shape coupling while retaining the behavioral contracts for preserving a sampled-set False result and omitting process-set evidence when identities were never captured.

Current branch boundary

After that late-failure slice, this same branch also gained the forced-close process-termination foundation now used as the exact prerequisite for PR #146. The current delta therefore includes the controlled forced-close process-termination contract and runner support in addition to the original late-failure evidence preservation. PR #146 remains the descendant lane for retaining that forced-close teardown evidence through its reviewed failure envelope; no descendant behavior is promoted into this PR by inference.

Current exact-head proof at 5ee76849a1dc22f10ddcd291c8bac4cd612fa6a1:

• CI 31692576373: success;
• Manifest V3 Compatibility 31692576338: success;
• GitHub reports the Draft mergeable against exact prerequisite test(browser): require sampled Chromium process-set termination evidence #144;
• no formal reviews, requested reviewers, or inline review threads are currently returned.

Truth boundary

When a controlled Agent Task fails after the bounded Chromium root-plus-descendant PID/start-time identities have been captured, shutdown waits for and preserves that exact sampled process-set termination outcome as well as browser-root termination. Failures before process-set capture do not invent descendant evidence. The additional forced-close foundation remains bounded Linux /proc PID/start-time evidence. None of this attests cgroup/task ownership, processes created only after sampling, cross-platform teardown, or OS-wide orphan absence.

This PR remains Draft behind its dependency stack and does not transfer prerequisite, descendant, review, or merge authority.

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