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/mainline-cinnamon/packages.list

@@ -0,0 +1,171 @@
+# Packages installed into the target system for the mainline-cinnamon profile.
+# Identical to stable-cinnamon but uses Linux mainline kernel (kernel 7) for
+# cutting-edge hardware support. Includes alsa-ucm-conf for SoundWire audio.
+# Lines beginning with '#' or empty are skipped.
+
+# --- base / boot ---
+base-system
+linux-mainline
+linux-mainline-headers
+linux-firmware
+linux-firmware-network
+intel-ucode
+grub-x86_64-efi
+efibootmgr
+os-prober
+dracut
+gptfdisk
+parted
+btrfs-progs
+dosfstools
+
+# --- core userspace ---
+sudo
+bash
+bash-completion
+git
+curl
+wget
+vim
+nano
+htop
+tmux
+unzip
+zip
+xz
+rsync
+pciutils
+usbutils
+lsof
+strace
+file
+which
+man-pages
+mdocml
+ca-certificates
+xtools
+
+# --- networking ---
+NetworkManager
+NetworkManager-openvpn
+openssh
+iwd
+wpa_supplicant
+nftables
+chrony
+wireless-regdb
+
+# --- audio (pipewire stack + SoundWire / SOF support for mainline) ---
+pipewire
+wireplumber
+alsa-pipewire
+pavucontrol
+alsa-utils
+alsa-ucm-conf
+sof-firmware
+
+# --- graphics / xorg ---
+xorg-minimal
+xorg-fonts
+xorg-input-drivers
+xf86-input-libinput
+xf86-video-intel
+mesa-dri
+mesa-vulkan-intel
+intel-video-accel
+vulkan-loader
+
+# --- nvidia (PRIME offload) ---
+nvidia
+nvidia-libs-32bit
+nvidia-vaapi-driver
+
+# --- desktop ---
+cinnamon
+xdg-user-dirs
+xdg-utils
+xdg-desktop-portal
+xdg-desktop-portal-gtk
+gvfs
+gvfs-mtp
+gvfs-smb
+file-roller
+gnome-keyring
+seahorse
+network-manager-applet
+blueman
+bluez
+
+# --- display manager ---
+lightdm
+lightdm-gtk3-greeter
+
+# --- fonts ---
+noto-fonts-ttf
+noto-fonts-emoji
+noto-fonts-cjk
+liberation-fonts-ttf
+dejavu-fonts-ttf
+font-awesome6
+
+# --- containers ---
+docker
+docker-compose
+
+# --- terminal ---
+alacritty
+
+# --- gtk theming deps (for gruvbox theme) ---
+sassc
+gnome-themes-extra
+gtk-engine-murrine
+dconf
+dconf-editor
+
+# --- media / utilities ---
+vlc
+obs
+flameshot
+
+# --- nix package manager ---
+nix
+
+# --- zram / swap ---
+zramen
+
+# --- power / laptop ---
+tlp
+tlp-rdw
+acpi
+acpid
+upower
+brightnessctl
+
+# --- printing ---
+cups
+cups-filters
+cups-pk-helper
+ghostscript
+foomatic-db
+gutenprint
+hplip
+system-config-printer
+sane
+simple-scan
+
+# --- bluetooth ---
+bluez-alsa
+
+# --- backups / snapshots ---
+timeshift
+grub-btrfs
+inotify-tools
+
+# --- trackpad gestures ---
+libinput-gestures
+wmctrl
+xdotool
+python3-setproctitle
+
+# --- screenshots ---
+xclip