extras: add PowerShell greeting port, Rust gm-switch and Windows installers

- powershell_greeting: PowerShell port of fish_greeting (fastfetch random logo, sixel/kitty-direct per terminal, logs, modes)

- win-gm-switch: Rust port of gm-switch.cpp (cross-platform mode file, --status, Start Menu shortcuts installer)

- install-windows.ps1 umbrella installer; README pointers

extras/install-windows.ps1

@@ -0,0 +1,59 @@
+# Install the J621 Windows extras (PowerShell greeting + Rust gm-switch).
+#
+# Usage:
+#   .\install-windows.ps1
+#   .\install-windows.ps1 -Url https://j621.example.ts.net -Token <token>
+#   .\install-windows.ps1 -NoPrompt
+#   .\install-windows.ps1 -Uninstall
+#
+# Options are forwarded to the component installers (see
+# powershell_greeting\install.ps1 and win-gm-switch\install.ps1).
+
+[CmdletBinding()]
+param(
+    [string]$Url,
+    [string]$Token,
+    [switch]$NoPrompt,
+    [switch]$Force,
+    [switch]$NoProfilePatch,
+    [switch]$SkipGreeting,
+    [switch]$SkipGmSwitch,
+    [switch]$Uninstall
+)
+
+$ErrorActionPreference = 'Stop'
+
+$here = $PSScriptRoot
+if (-not $here) { $here = Split-Path -Parent $MyInvocation.MyCommand.Path }
+$failures = New-Object System.Collections.Generic.List[string]
+
+if (-not $SkipGmSwitch) {
+    Write-Host ''
+    Write-Host '=== gm-switch ==='
+    $params = @{}
+    if ($Uninstall) { $params.Uninstall = $true }
+    & (Join-Path $here 'win-gm-switch\install.ps1') @params
+    if ($LASTEXITCODE -ne 0) { $failures.Add('gm-switch installer failed') }
+}
+
+if (-not $SkipGreeting) {
+    Write-Host ''
+    Write-Host '=== PowerShell greeting ==='
+    $params = @{}
+    if ($Uninstall) { $params.Uninstall = $true }
+    if ($Url) { $params.Url = $Url }
+    if ($Token) { $params.Token = $Token }
+    if ($NoPrompt) { $params.NoPrompt = $true }
+    if ($Force) { $params.Force = $true }
+    if ($NoProfilePatch) { $params.NoProfilePatch = $true }
+    & (Join-Path $here 'powershell_greeting\install.ps1') @params
+    if ($LASTEXITCODE -ne 0) { $failures.Add('greeting installer failed') }
+}
+
+Write-Host ''
+if ($failures.Count -eq 0) {
+    Write-Host 'All done.'
+    exit 0
+}
+foreach ($failure in $failures) { Write-Host $failure }
+exit 1