docs(research): close Windows containment unknowns by live experiment (#258)

[Sandstorm831]

Follow-up research for Establish how Secant can contain a Harness's descendant processes on Windows. It updates docs/research/windows-process-containment.md in place, closing its Still Unknown list by live experiment on Windows 11 with Claude Code 2.1.283 and codex-cli 0.155.0.

• Codex's Windows sandbox. Git Bash can't run in it, an outer job still contains a sandboxed command, and turn/interrupt still leaves the command running.
• Codex PTY mode. The kill comes from the pseudoconsole closing, not the job.
• Job chains and the gap. It maps the job chains, and measures the assign-after-spawn gap as never populated (0 of 20 runs per Harness).
• Git Bash cases. More cases are recorded, and CLAUDE_CODE_GIT_BASH_PATH doesn't change the escape.
• Claude Code's interrupt. A raw interrupt still runs taskkill /T /F.
• Detached processes. A no-breakaway job kills them, and CREATE_BREAKAWAY_FROM_JOB fails with error 5 inside it.
• A contained spawn with streamed stdio. It uses bun:ffi CreateProcessW with PROC_THREAD_ATTRIBUTE_JOB_LIST, with stdio carried over node:net named pipes. Live Claude Code and Codex were contained 3 of 3 each, and so was a parent crash.

Facts only. The choice belongs to Decide how Secant contains a Harness's descendant processes on Windows. Merge with a merge commit (not squash or rebase) so the pinned links in the resolution comments survive.

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