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

tools/CONTRACT.md

@@ -65,6 +65,11 @@ From PowerShell 7 on Windows, the twin: `pwsh -NoProfile -ExecutionPolicy Bypass
 
 Until it has passed, every `tools/wikitool` call exits 42 and names it.
 
+A release also carries both scripts as assets (`preflight.sh`, `preflight.ps1`) for the very
+first install, before there is a tree: run from a folder with no `tools/prerequisites.txt` beside
+them, they download and verify the release tarball, unpack it into `chemenu/` next to themselves
+(or `--into <path>`), and run the preflight inside the unpacked tree.
+
 ## Usage
 
 Run from the repo root: