Bisect: try minimal .iss before our real one

The cwd-sync + temp-iss + /O fixes didn't move the needle - ISCC still
prints its full banner (Russell + Laan + Yackimoff) then dies with
"system cannot find the path specified." after 225ms with exit 2.
That rules out cwd, /D, OutputDir-resolution, and DLL-load failures.

Compile a truly minimal .iss (no #defines, no [Code], no [Files],
CreateAppDir=no, Uninstallable=no) right before our real one. If the
minimal compiles cleanly, the problem is in our .iss content and we
can bisect from there. If even the minimal fails, ISCC is broken
under the runner's SYSTEM context regardless of input - then the fix
is to stop building under SYSTEM (run the runner under a regular
user account, or skip ISCC altogether and ship the bare exe + scripts
as a zip).

scripts/build-installer.ps1

@@ -142,6 +142,31 @@ try {
     Write-Host "  PS  location (post): $((Get-Location).Path)"
     Write-Host "  .NET cwd     (post): $([System.IO.Directory]::GetCurrentDirectory())"
 
+    # Sanity: compile a minimal .iss right next to ours BEFORE attempting the
+    # real one. Minimal has no #defines, no [Code], no [Files], no compression
+    # tweak - just the absolute floor of what ISCC will accept. If THIS fails
+    # under the same SYSTEM context with the same identical exit/error, the
+    # problem is environmental, not in our .iss content.
+    $minIss = Join-Path $issDir "min-test.iss"
+    @"
+[Setup]
+AppName=MinTest
+AppVersion=1.0
+AppId={{12345678-1234-1234-1234-123456789ABC}
+DefaultDirName={pf}\MinTest
+CreateAppDir=no
+Uninstallable=no
+OutputBaseFilename=mintest
+OutputDir=$dist
+"@ | Set-Content -Path $minIss -Encoding ascii
+    Write-Host ""
+    Write-Host "--- bisect step 1: minimal .iss ---" -ForegroundColor Cyan
+    & $iscc (Split-Path $minIss -Leaf) *>&1 | ForEach-Object { Write-Host "  $_" }
+    $minExit = $LASTEXITCODE
+    Write-Host "  minimal exit: $minExit"
+    Remove-Item $minIss -ErrorAction SilentlyContinue
+    Write-Host ""
+
     # Bake the version into a temp .iss and override OutputDir to an absolute
     # path so nothing in the build depends on cwd resolution.
     $tempIss = Join-Path $issDir "webhook-server.gen.iss"