Harden offline research corpus preflight and feature evidence

[sodejm]

Problem and change

Offline research preflight could reject a frozen plan with an empty argument tuple and could block while opening a substituted FIFO corpus. Accept the immutable empty tuple, open corpus sources without waiting on a FIFO, and reject nonregular sources before staging. Add executable feature scenarios for the supported fixture behavior and document the limits of that evidence.

Refs #245. This increment does not close the issue: native Linux denied-egress, credential, socket, canary, and cleanup evidence is still required for the full acceptance criteria.

Validation and review

• Focused offline research and feature-step tests: 21 passed.
• Repository push gate on 62adf5a2fee10c1b46f4338322ce13772c9aadc6: passed make check, pinned Ruff checks, 2,196 pytest cases (20 skipped), coverage, and pip-audit.
• Independent review found a fixture-evidence wording overclaim; the feature scenario wording was narrowed before this PR.
• Native Linux isolation was not exercised in this checkout. The issue remains open for that evidence.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-15T05:04:01.857271Z	62adf5a	PR opened

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