test: collect first-run home and network design evidence

[Sunrisepeak]

This temporary draft collects evidence for the first-run home and network design in #822. It contains only observational tests and their Actions workflow; it does not implement the proposed features and is not intended for merge.

The Linux, Windows and macOS probes use the checksum-verified 2026.10.17.1 release. They assert existing explicit-home, self-contained, fallback, offline-doctor and force-cleanup behavior, and record cold doctor/config under a refused loopback proxy with an external 12-second deadline. A macOS LLVM control compares direct imports, transitive imports and imports reached through a header for #816 using a local dependency.

Validation: the Linux home/offline/force baseline ran locally. Full per-host observations and logs are uploaded as Actions artifacts. A green observational job does not mean that a timed-out product command passed a bounded-network contract. New home redirection and the Windows prompt remain design proposals.

Related benchmark evidence: https://github.com/mcpp-community/mcpp/actions/runs/39129581254/job/117682786534

Final probe run: https://github.com/mcpp-community/mcpp/actions/runs/39184402622 (a736c64). All four dedicated jobs completed successfully.

Observed product behavior:

• Explicit MCPP_HOME, portable resolution, fallback resolution, JSON-env read-only behavior and existing force cleanup passed on Linux, Windows and macOS.
• Offline doctor completed in 0.565 s (Linux), 1.094 s (Windows), and 0.722 s (macOS). Under the refused proxy, ordinary doctor exceeded the external 12-second observation window on all three hosts. Linux and Windows config also exceeded it; macOS config returned after about 12 seconds despite ninja bootstrap failure.
• LLVM 23.1.3 on macOS built both direct and source-level transitive imports. The header-transitive case failed to find depmod and its main compilation lacked a modmap. This is an observed defect, not a product pass; the observational job is green because the direct control succeeded and evidence was collected.

The first module-control attempt invoked a workspace shim and is invalid as product evidence. The second attempt selected a checksum-verified real binary. The general CI workflow, automatically triggered by these test-only files, was cancelled; no claim of a full CI pass is made. This draft remains unmerged.