fix(installer): stop PS 5.1 stderr wrapping from aborting version probes

Distilled from #1255 by @MasterFede5, rebased onto the single-binary
installer and completed with the two review conditions.

PowerShell 5.1 wraps redirected native stderr into ErrorRecords, so under
the script's global ErrorActionPreference=Stop a HEALTHY binary that
prints a single warning to stderr while exiting 0 aborts both
`--version 2>&1` probes with "downloaded binary failed to run". Both
probes now relax EAP to Continue for the probe call only, with an
exception-safe restore; failure detection stays on $LASTEXITCODE.

The review-required pre-seed ($global:LASTEXITCODE = 1) closes the window
the relaxation would otherwise open: a binary that fails to START under
Continue may leave $LASTEXITCODE untouched, and the installed-binary probe
runs after earlier successful native calls, so a stale 0 would have turned
a corrupt install into "Installed: ...". Seeding 1 makes start-failure
read as failure on both probes.

The bundle contract pins the guard's two load-bearing parts (the scoped
relaxation with restore, and the pre-seed), verified failing when either
is stripped. The change adds zero non-ASCII bytes; the file remains pure
ASCII per the PS 5.1 ANSI-decode rule.

Co-authored-by: MasterFede5 <friedrickms@gmail.com>
Signed-off-by: Martin Vogel <martin.vogel.tech@gmail.com>

install.ps1

@@ -281,8 +281,21 @@ if ($binaryItem.Attributes -band [System.IO.FileAttributes]::ReparsePoint) {
 }
 
 # Prove the downloaded binary runs before touching an existing installation.
+# PS 5.1 wraps redirected native stderr into ErrorRecords, so under the global
+# ErrorActionPreference=Stop a HEALTHY binary that prints one warning while
+# exiting 0 becomes a terminating error here. Relax to Continue for the probe
+# only; failure detection stays on $LASTEXITCODE, and the pre-seed guarantees
+# a binary that fails to START (stale $LASTEXITCODE from an earlier native
+# call) can never read as success.
 try {
-    $candidateVersion = & $DownloadedBinary --version 2>&1
+    $ProbeEap = $ErrorActionPreference
+    try {
+        $ErrorActionPreference = "Continue"
+        $global:LASTEXITCODE = 1
+        $candidateVersion = & $DownloadedBinary --version 2>&1
+    } finally {
+        $ErrorActionPreference = $ProbeEap
+    }
     if ($LASTEXITCODE -ne 0) { throw "candidate exited with $LASTEXITCODE" }
     Write-Host "Verified candidate: $candidateVersion"
 } catch {
@@ -349,8 +362,18 @@ if (Test-Path -LiteralPath $DownloadedInstaller -PathType Leaf) {
 }
 
 # Verify
+# Same PS 5.1 stderr-wrapping guard as the candidate probe above; the pre-seed
+# matters MOST here, because prior successful native calls leave a stale
+# $LASTEXITCODE=0 that a start-failure would otherwise inherit.
 try {
-    $ver = & $Dest --version 2>&1
+    $ProbeEap = $ErrorActionPreference
+    try {
+        $ErrorActionPreference = "Continue"
+        $global:LASTEXITCODE = 1
+        $ver = & $Dest --version 2>&1
+    } finally {
+        $ErrorActionPreference = $ProbeEap
+    }
     if ($LASTEXITCODE -ne 0) { throw "installed binary exited with $LASTEXITCODE" }
     Write-Host "Installed: $ver"
 } catch {

tests/test_windows_bundle_contract.sh

@@ -229,6 +229,19 @@ require(
     "install.ps1 must verify and install through the downloaded binary",
 )
 
+# PS 5.1 wraps redirected native stderr into ErrorRecords; under the global
+# ErrorActionPreference=Stop a healthy binary that warns on stderr aborts the
+# version probes (#1255). Pin the guard's two load-bearing parts so neither
+# can be silently dropped: the probe-scoped relaxation with an exception-safe
+# restore, and the $LASTEXITCODE pre-seed that keeps a start-failure from
+# inheriting a stale 0 and reading as success.
+require(
+    installer.count('$ErrorActionPreference = "Continue"') == 2
+    and installer.count("$global:LASTEXITCODE = 1") == 2
+    and installer.count("$ErrorActionPreference = $ProbeEap") == 2,
+    "install.ps1 version probes must relax EAP with restore and pre-seed LASTEXITCODE (#1255)",
+)
+
 # ── 4. Package-manager shims resolve the single Windows binary ───────────────
 single_binary_contracts = {
     "pkg/npm/install.js": (