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/).

installer/install.sh

@@ -35,12 +35,18 @@ export PKG_LIST_FILE
 [[ -r "$PKG_LIST_FILE" ]] || die "packages.list $PKG_LIST_FILE missing"
 
 # ---------- profile ----------
+# PROFILE can be set on the command line or in the environment.
+# If not set, fall back to DEFAULT_PROFILE which the live ISO writes to
+# /etc/profile.d/00-void-installer.sh so the embedded installer defaults to
+# the same configuration the live session was built with.
+PROFILE="${PROFILE:-${DEFAULT_PROFILE:-stable-cinnamon}}"
+export PROFILE
 PROJECT_DIR="${PROJECT_DIR:-$SHARE_DIR}"
 PROFILES_DIR="${PROFILES_DIR:-$SHARE_DIR/profiles}"
 export PROJECT_DIR PROFILES_DIR
 # shellcheck source=lib/profiles.sh
 source "$INSTALLER_DIR/lib/profiles.sh"
-load_profile || die "could not load profile '${PROFILE:-stable-cinnamon}'"
+load_profile || die "could not load profile '${PROFILE}'"
 # Profile may override the package list.
 [[ -r "$PROFILE_PACKAGES_FILE" ]] && PKG_LIST_FILE="$PROFILE_PACKAGES_FILE"
 log "using packages list: $PKG_LIST_FILE"