feat(template): add -SkipTier2 switch to skip Tier-2 toolchain installation

Setup-WinBuild2025.ps1:
  - New [switch] \ param (after \)
  - Step 12 block wrapped with if (\) { skip-message } else { ... }
  - Final gate Tier-2 Assert-Step wrapped with if (-not \)

Prepare-WinBuild2025.ps1:
  - New [switch] \ param forwarded to Setup via setupArgs

Validate-SetupState.ps1:
  - New [switch] \ param
  - \ passed into both remote scriptblocks via -ArgumentList
  - Tier-2 Chk calls guarded with if (-not \) in both blocks

template/Prepare-WinBuild2025.ps1

@@ -141,6 +141,9 @@ param(
     [string] $GhCliVersion   = '2.67.0',
     [string] $VcpkgRef       = '',
 
+    # Skip Tier-2 Toolchain — forwarded to Setup-WinBuild2025.ps1 -SkipTier2
+    [switch] $SkipTier2,
+
     [switch] $SkipWindowsUpdate,
 
     # Skip disk cleanup inside the VM (cleanmgr + DISM + cache clear) — speeds up re-runs
@@ -386,6 +389,7 @@ try {
     }
     if ($SkipWindowsUpdate) { $setupArgs['SkipWindowsUpdate'] = $true }
     if ($SkipCleanup)       { $setupArgs['SkipCleanup']       = $true }
+    if ($SkipTier2)         { $setupArgs['SkipTier2']         = $true }
 
     # ── Run Setup-WinBuild2025.ps1 inside the VM ──────────────────────────────
     # Setup may return exit 3010 (ERROR_SUCCESS_REBOOT_REQUIRED) when Windows