Back up User PATH inside Add-ToUserPath, before first mutation

Previously only studio/setup.ps1 took a one-time PATH backup, at script
top (line ~547). install.ps1 (the irm | iex entry point) had no backup,
so users who installed via that path had no recovery surface if anything
clobbered their PATH. The PR description's "one-time backup before any
modifications" promise only held for the studio installer flow.

Move the backup into Add-ToUserPath itself: just before the first actual
SetValue mutation, write the pristine raw PATH to
HKCU\Software\Unsloth\PathBackup if no backup already exists. This:

- Covers both entry points (install.ps1 and studio/setup.ps1).
- Captures the TRUE pristine PATH even when install.ps1 runs first and
  studio/setup.ps1 runs afterwards (the script-top backup in setup.ps1
  would otherwise see an already-modified PATH).
- Is idempotent: once a backup exists, subsequent calls preserve it.
- Skips when nothing would mutate (dedup match) or PATH is empty.

The script-top backup in studio/setup.ps1 is kept for defense in depth.

install.ps1

@@ -142,6 +142,22 @@ function Install-UnslothStudio {
                         return $false  # already present
                     }
                 }
+                # One-time backup of the pristine User PATH before our first
+                # mutation. Stored under HKCU\Software\Unsloth so a wiped/clobbered
+                # PATH can be recovered. Idempotent: existing backup is preserved.
+                if ($rawPath) {
+                    try {
+                        $backupKey = [Microsoft.Win32.Registry]::CurrentUser.CreateSubKey('Software\Unsloth')
+                        try {
+                            $existingBackup = $backupKey.GetValue('PathBackup', $null)
+                            if (-not $existingBackup) {
+                                $backupKey.SetValue('PathBackup', $rawPath, [Microsoft.Win32.RegistryValueKind]::ExpandString)
+                            }
+                        } finally {
+                            $backupKey.Close()
+                        }
+                    } catch { }
+                }
                 if (-not $rawPath) {
                     Write-Host "[WARN] User PATH is empty — initializing with $Directory" -ForegroundColor Yellow
                 }

studio/setup.ps1

@@ -112,6 +112,24 @@ function Add-ToUserPath {
                     return $false  # already present
                 }
             }
+            # One-time backup of the pristine User PATH before our first
+            # mutation. Stored under HKCU\Software\Unsloth so a wiped/clobbered
+            # PATH can be recovered. Idempotent: existing backup is preserved.
+            # The script-top backup at line ~547 covers the studio entry point;
+            # this in-helper backup also covers callers that bypass that block.
+            if ($rawPath) {
+                try {
+                    $backupKey = [Microsoft.Win32.Registry]::CurrentUser.CreateSubKey('Software\Unsloth')
+                    try {
+                        $existingBackup = $backupKey.GetValue('PathBackup', $null)
+                        if (-not $existingBackup) {
+                            $backupKey.SetValue('PathBackup', $rawPath, [Microsoft.Win32.RegistryValueKind]::ExpandString)
+                        }
+                    } finally {
+                        $backupKey.Close()
+                    }
+                } catch { }
+            }
             if (-not $rawPath) {
                 Write-Host "[WARN] User PATH is empty — initializing with $Directory" -ForegroundColor Yellow
             }