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

CHANGES.md

@@ -59,7 +59,7 @@ concern - readable here, never shipped as something to parse.
 
 ---
 
-## 8.0.0-beta.15 - 2026-10-01 - PowerShell 7 preflight and launcher: tools/preflight.ps1, tools/wikitool.ps1, doctor checks for execution policy and Mark of the Web
+## 8.0.0-beta.16 - 2026-10-01 - Preflight as a release asset: download, verify and unpack the stack, then run the tree preflight
 
 **Author:** Torben Nehmer
 
@@ -90,6 +90,7 @@ concern - readable here, never shipped as something to parse.
 - publish: the gate lists the staged state; a missing or unreachable remote stops before the commit
 - Path budget: a file's path stays at 160 characters or fewer so a Windows checkout works without long paths (#163)
 - PowerShell 7 preflight and launcher: tools/preflight.ps1, tools/wikitool.ps1, doctor checks for execution policy and Mark of the Web
+- Preflight as a release asset: download, verify and unpack the stack, then run the tree preflight
 
 **Low impact**
 - version bump no longer points at version release in its output
@@ -126,6 +127,40 @@ concern - readable here, never shipped as something to parse.
 - raw/CONTRACT.md points at the path budget for a name accepted from incoming/
 <!-- /wikitool:bumps -->
 
+### Preflight as a release asset: download, verify and unpack the stack, then run the tree preflight
+
+Section C of #151, the first install. Until now a new user had to get the stack onto the machine
+before any preflight could run - which is exactly the step that fails on a machine without git
+or a short enough path. Each release now also attaches `preflight.sh` and `preflight.ps1` as
+assets, and a script without `tools/prerequisites.txt` beside it runs in **asset mode**: it
+downloads the release tarball and its `.sha256`, stops with exit 1 unless the checksum matches,
+reads the folder limit from `tools/prerequisites.txt` inside the archive, and unpacks into
+`chemenu/` next to itself (`--into <path>` for another place). It unpacks into a temporary
+sibling and renames, requires exactly one top-level folder, and refuses an existing target
+without touching it. Then it runs the unpacked tree's own preflight, passing `--set` and the exit
+code through, so everything after the unpack is the tree mode that already existed.
+
+The asset copies are made by `release.yml`: it writes the same release's tarball and checksum
+URLs into two placeholder lines of each script, checks that the substitution took, and uploads
+the two files under exactly those names. The tree copies keep the placeholders empty; an asset
+script with empty placeholders and no `--archive` exits 1 saying it does not come from a release.
+`--archive <tarball>` uses a tarball already on disk (its `<tarball>.sha256` must sit beside it)
+for a machine that cannot download, and is the entry point the tests use.
+
+On POSIX asset mode needs `curl`, `tar` and `sha256sum` (or `shasum`) and stops with exit 42 and
+the usual guidance block when one is missing; the PowerShell script uses what Windows ships plus
+`tar`. On Windows with long paths off, the 95-character folder limit is judged at the final
+target before anything is unpacked, so a too-long `--into` stops with exit 42 and the fix is a
+shorter folder. Git Bash gets the target and archive converted with `cygpath -u`, because GNU
+tar would read `C:` as a host.
+
+The tests build a release tarball with a checksum and run both scripts against it, including a
+local HTTP server for the PowerShell download, the 95/96 boundary, a failed checksum, two
+top-level folders, an existing target, and a test that ties the placeholder lines to the `sed`
+expressions in `release.yml`. `tools/CONTRACT.md`, `tools/README.md`, `README.md`,
+`instructions/preflight.md` (a new decision point for the asset script),
+`instructions/dev/testing-conventions.md` and `INSTALL.md` describe the first-install route.
+
 ### PowerShell 7 preflight and launcher: tools/preflight.ps1, tools/wikitool.ps1, doctor checks for execution policy and Mark of the Web
 
 The PowerShell half of #151. Harnesses that run in PowerShell 7 on Windows (GitHub Copilot CLI,