test: probe runtime cancellation and process ownership

[skevetter]

The next host-hardening spike exercises real host/plugin/runtime-child ownership through the SDK's go-plugin/gRPC transport. The fixture launches a blocking child, acknowledges readiness before cancellation, answers independent liveness probes, and reports reaping only after exec.Cmd.Wait returns.

The probes record two ownership gaps that must be resolved before runtime cutover:

• Abrupt plugin death leaves the runtime child running.
• Abrupt host death cancels the RPC and lets a cooperative plugin reap its child, but leaves the plugin serving.

Unary and Exec cancellation both reap the fixture child, including a child that ignores interruption. Unix asserts that the child received and ignored the signal before forced termination. Windows exercises the bounded forced-kill fallback because Go cannot deliver os.Interrupt to its child process there.

Surviving processes are independently killed by the test observer, with retained process handles, and crash-left socket files are isolated in a short private temporary directory. The README explains the results and their limits. No public supervisor API or host lifetime policy is introduced; passing these tests records the transport's current gaps rather than certifying abrupt process-tree cleanup.

CI keeps the existing Linux/macOS/Windows race-test matrix and publishes runtime-tests-* JSON artifacts for reviewable platform evidence. Bash pipe failure propagation preserves test failures when teeing the report.

Local validation: full SDK race tests, vet, strict lint, prek, and diff checks passed. Local CodeRabbit identified an observer teardown backpressure issue; the fix was re-reviewed with zero findings. The last successful fresh committed review covered all six files with zero findings. A small event-routing extraction then brought the reader back within strict complexity limits; its final fresh CodeRabbit rerun was rate-limited (49-minute reset). The final source is validated with race tests, vet, and prek; the uncommitted workspace check is clean.

CI validation: run 37130756350 passed all three platform race/vet jobs, all startup probes, generated bindings, Lint, and Pre-commit. The saved JSON reports confirm all four cancellation combinations and both ownership-gap scenarios on Linux, macOS, and Windows. Release Please was skipped as expected for a pull request.

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• .github/workflows/ci.yml
• README.md
• internal/processfixture/main.go
• internal/processfixture/observer.go
• internal/processprobe/observer_test.go
• internal/processprobe/process_test.go

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