feat: preflight as a release asset - download, verify, unpack, then run the tree preflight (#151, C)

Files changed:
- .gitea/workflows/release.yml
- CHANGES.md
- INSTALL.md
- README.md
- VERSION
- instructions/dev/testing-conventions.md
- instructions/preflight.md
- tools/CONTRACT.md
- tools/README.md
- tools/chemenu/tests/test_preflight.py
- tools/chemenu/tests/test_preflight_pwsh.py
- tools/preflight.ps1
- tools/preflight.sh

instructions/dev/testing-conventions.md

@@ -165,6 +165,11 @@ Whenever you add or change a test under `tools/chemenu/tests/`.
 
 ## Decision points
 
+- **Testing a PowerShell script?** `test_preflight_pwsh.py` needs `pwsh` and skips silently
+  without it - a green run on a machine with no PowerShell 7 has not run those tests. Check
+  `command -v pwsh` before trusting the result; CI's `pwsh` job runs them in the image built from
+  `.gitea/pwsh-ci/Dockerfile`, and so can you (`docker run` that image with the checkout mounted
+  and `pytest tools/chemenu/tests/test_preflight_pwsh.py` as the command).
 - **A test genuinely needs the developer's real environment?** There is no such test, and a new
   one is a design problem rather than an exception: what it wants is a fixture that *builds*
   the state it needs inside `tmp_path`. Building it is also the only version CI can run.