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/win-gm-switch/README.md

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+# win-gm-switch (Rust)
+
+Rust port of the old `gm-switch.cpp` from
+`J621-Django/extras/fish_greeting system/`: a tiny CLI that sets the J621
+shell greeting rating mode (0 = NSFW, 1 = SFW, 2 = Questionable).
+
+## Usage
+
+```
+gm-switch --mode 0      # NSFW (explicit)
+gm-switch --mode 1      # SFW (safe)
+gm-switch --mode 2      # Questionable
+gm-switch --status      # print the current mode
+```
+
+`--path <file>` overrides the mode file (also via the `J621_MODE_FILE`
+environment variable). Invalid input prints the same usage/errors as the C++
+tool and exits 1.
+
+Mode file locations:
+
+- **Windows**: `%APPDATA%\j621Greeting\greeting_mode` - read by
+  [`../powershell_greeting/`](../powershell_greeting/).
+- **Linux/macOS**: `~/.config/fish/greeting_mode` - read by
+  [`../fish_greeting/`](../fish_greeting/), so this binary can stand in for
+  the C++ tool there.
+
+## Install (Windows)
+
+```powershell
+cd extras\win-gm-switch
+.\install.ps1
+```
+
+The installer builds with `cargo build --release`, installs `gm-switch.exe`
+to `%LOCALAPPDATA%\J621\bin`, adds that folder to your user PATH (new
+terminals pick it up) and creates Start Menu shortcuts under **J621
+Greeting** (NSFW / SFW / Questionable). Each shortcut opens a small console
+showing the confirmation; close it with `exit` or the window button.
+
+Uninstall:
+
+```powershell
+.\install.ps1 -Uninstall
+```
+
+## Build and test without installing
+
+```powershell
+cargo build --release   # -> target\release\gm-switch.exe
+cargo test
+```
+
+Zero runtime dependencies (std only). The `target\` folder is gitignored.