fix(response): validate and terminate through one process handle

[Karib0u]

Summary

Active response could validate one process and then kill an unrelated replacement if the PID was reused before termination. Keep the process reference alive through validation and termination.

• Linux opens a pidfd before reading identity, confirms it is still live after the /proc reads, and sends SIGKILL through that same pidfd. Unsupported or denied pidfd operations fail safely without a bare-PID fallback.
• Windows opens one handle with query and terminate rights, queries identity through it, and terminates through that same handle.
• Add normal-CI child-process and injected-swap tests, and document each platform's guarantee, including the remaining macOS PID-reuse window.

Fixes #601.

Type of change

• bug - bug fix

Test plan

• Native Windows tests: not run locally. Windows GNU cross-check and all-targets Clippy passed.
• Linux: 902 tests passed, 16 normally ignored, with the embedded eBPF ABI check excluded because the local build uses RUSTINEL_EBPF_STUB=1.
• macOS: full suite passed (836 tests), plus all 7 active-response integration tests including the previously ignored live tests.
• New tests run without an example build or elevated privileges. The focused Linux run passed as UID 1000.
• Regression verification: a temporary bare-PID termination mutation killed the replacement and failed the swap test; the retained-handle implementation was restored and the focused suite passed.
• Clippy passed on macOS, Linux, and the Windows GNU cross target. Formatting, documentation lint/reflow, and diff checks passed.

Checklist

• Labels added: bug, security, linux, windows.
• Active-response documentation updated with platform guarantees and failure behavior.
• Generated documentation and release preparation checks are not applicable.