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/profile.conf

@@ -0,0 +1,28 @@
+# Stable-niri profile.
+# Void's stable LTS kernel (linux, kernel 6) + niri Wayland tiling compositor + noctalia-shell.
+PROFILE_NAME="stable-niri"
+PROFILE_DESC="Stable Void kernel + niri Wayland (scrolling tiler) + noctalia-shell"
+
+# Kernel — use Void's stable LTS.
+KERNEL_PKG="linux"
+
+# Display server / DE.
+DISPLAY_SERVER="wayland"
+DESKTOP="niri"
+
+# noctalia-shell via third-party XBPS repo (universalrepo.r1xelelo.workers.dev).
+WAYLAND_SHELL="noctalia"
+
+# Package list (relative to repo root).
+PROFILE_PACKAGES_FILE="config/profiles/stable-niri/packages.list"
+
+# Theme settings (gtk3/gtk4 apps under wayland read these).
+GTK_THEME="Gruvbox-Dark"
+ICON_THEME="Gruvbox-Plus-Dark"
+DEFAULT_TERMINAL="alacritty"
+CURSOR_THEME="Bibata-Modern-Ice"
+
+# Wayland env defaults (exported into /etc/environment by profile customisation).
+QT_QPA_PLATFORM="wayland;xcb"
+GDK_BACKEND="wayland,x11"
+MOZ_ENABLE_WAYLAND="1"