Assert the PowerShell floor install.ps1 already documents

[MongLong0214]

Closes #703.

install.ps1:44 already says it:

Windows PowerShell 5.1 and PowerShell 7+ both run this. Nothing here uses the
ternary operator, null-coalescing, or any other 7-only syntax.

Measured, and true today — ??, ?., &&/||, ternary, -Parallel,
-AsHashtable: zero each. Nothing enforces it.

The intent is written where a reader looks and the enforcement is nowhere. Same
shape as everything else found here this week.

Why this floor is worth asserting rather than discovering

PowerShell parses the whole file before executing any of it. A single ?? fails
at parse time, so the installer's own diagnostics never run — a 5.1 user would
not see Node.js 22.23.2 or newer is required or Nothing was installed. They
would see a syntax error from a file they did not write, about a version nobody
told them they needed.

CI does run install.ps1 under both powershell.exe and pwsh. That proves it
works on a day someone maintains those jobs. This proves it on the day someone
writes ??.

Details

Comments are excluded from the scan, so the file can keep naming these
constructs while explaining why it avoids them — which is what line 45 does. The
second assertion pins that the file still states the floor in prose, because a
contributor who does not know 5.1 is supported cannot keep it.

Verified by injection:

$x = $a ?? $b                                  → 1 failed
$y = ($a) ? 1 : 2                              → 1 failed
Get-Item | ForEach-Object -Parallel { $_ }     → 1 failed
restored                                       → 7 passed

Test-only; no code, no artifact change.

[github-actions]

CommitLore — record lint

Trailers: clean — 1 commit in origin/main..715fbc01ff1ae03cf27b6b1d155239bfe23cc4ea
Active constraints: not read — commitlore: the index is incomplete: the build stopped after its time budget with 742 commit(s) or note(s) unread — records in them are missing from this answer. fix: commitlore init (or commitlore index) to finish the index (1 changed path)

_{Trailer violations fail this check. Active constraints are informational — they are what the repository already decided, not a verdict on this PR.}