refactor: unified multi-profile build system

Add 4 configurable profiles (stable-cinnamon, stable-niri,
mainline-cinnamon, mainline-niri) with a single unified build
entry point.

- iso/build.sh: replaces build-iso.sh / build-live-iso.sh /
  build-niri-live-iso.sh; accepts --profile and --type flags
- iso/_inner-build-unified.sh: replaces the three _inner-build-*.sh
  scripts; branches on BUILD_TYPE / DESKTOP / KERNEL_PKG env vars
- config/profiles/stable-niri/: new — linux (k6) + niri/Wayland/noctalia
- config/profiles/mainline-cinnamon/: new — linux-mainline (k7) + Cinnamon/X11
- config/profiles/mainline-niri/packages.live.list: symlink added
- config/profiles/stable-cinnamon/packages.live.list: symlink added
- Makefile: PROFILE variable (default stable-cinnamon), shellcheck updated
- installer/install.sh: respects DEFAULT_PROFILE env (set by live ISO)
- tests/run-qemu-test.sh: passes PROFILE through to build and overlay

Live ISOs embed the installer pre-configured for the same profile
they were built with (DEFAULT_PROFILE in /etc/profile.d/).

config/profiles/stable-niri/packages.live.list

@@ -0,0 +1,135 @@
+# Packages included in the LIVE desktop ISO squashfs for the stable-niri profile.
+# Boots into a niri/noctalia session. Single kernel (Void's stable LTS — no
+# dual-kernel menu needed here; use mainline-niri if you want kernel 7 + fallback).
+# Lines beginning with '#' or empty are skipped.
+
+# --- base / boot ---
+base-system
+linux
+linux-headers
+linux-firmware
+linux-firmware-network
+intel-ucode
+dracut
+
+# --- core userspace ---
+sudo
+bash
+bash-completion
+git
+curl
+wget
+vim
+nano
+htop
+tmux
+unzip
+zip
+xz
+rsync
+pciutils
+usbutils
+lsof
+file
+which
+man-pages
+mdocml
+ca-certificates
+xtools
+gptfdisk
+parted
+btrfs-progs
+dosfstools
+efibootmgr
+
+# --- networking ---
+NetworkManager
+NetworkManager-openvpn
+openssh
+iwd
+chrony
+wireless-regdb
+
+# --- audio (pipewire stack + SoundWire / SOF support) ---
+pipewire
+wireplumber
+alsa-pipewire
+pavucontrol
+alsa-utils
+alsa-ucm-conf
+playerctl
+sof-firmware
+
+# --- Wayland session ---
+mesa-dri
+niri
+xwayland-satellite
+elogind
+seatd
+dbus
+wayland
+
+# --- nvidia PRIME (from nonfree repo) ---
+nvidia
+nvidia-vaapi-driver
+
+# --- display manager ---
+greetd
+tuigreet
+
+# --- terminal + launcher ---
+alacritty
+
+# --- notification + background ---
+mako
+swaybg
+
+# --- bluetooth ---
+bluez
+blueman
+
+# --- polkit (auth dialogs) ---
+polkit
+polkit-gnome
+
+# --- noctalia-shell runtime deps ---
+brightnessctl
+ImageMagick
+python3
+upower
+power-profiles-daemon
+wl-clipboard
+zenity
+
+# --- XDG portals ---
+xdg-desktop-portal
+xdg-desktop-portal-gtk
+xdg-desktop-portal-gnome
+xdg-utils
+xdg-user-dirs
+
+# --- file manager ---
+nautilus
+
+# --- keyring ---
+gnome-keyring
+
+# --- nix (for prebaked packages) ---
+nix
+
+# --- noctalia-shell (from noctalia third-party XBPS repo) ---
+noctalia-shell
+
+# --- fonts ---
+noto-fonts-ttf
+noto-fonts-emoji
+noto-fonts-cjk
+liberation-fonts-ttf
+dejavu-fonts-ttf
+font-awesome6
+
+# --- gtk theming deps ---
+sassc
+gnome-themes-extra
+gtk-engine-murrine
+dconf