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/).

tests/run-qemu-test.sh

@@ -67,15 +67,17 @@ export TEST_PUBKEY
 # ---------- 2) build TEST ISO ----------
 TEST_ISO="$OUT_DIR/void-install-TEST.iso"
 TEST_OVERLAY_DIR="$OUT_DIR/test-overlay"
-"$PROJECT_DIR/tests/lib/make-test-overlay.sh" "$TEST_OVERLAY_DIR"
+TEST_PROFILE="${PROFILE:-stable-cinnamon}" \
+    "$PROJECT_DIR/tests/lib/make-test-overlay.sh" "$TEST_OVERLAY_DIR"
 
 if [[ ! -f "$TEST_ISO" || -n "${REBUILD_ISO:-}" ]]; then
-    blue "building test ISO -> $TEST_ISO"
+    blue "building test ISO -> $TEST_ISO  (profile: ${PROFILE:-stable-cinnamon})"
     EXTRA_INCLUDE_DIR="$TEST_OVERLAY_DIR" \
     OUTPUT_ISO="$TEST_ISO" \
     INSTALL_REPO_URL="http://10.0.2.2:3142/current" \
     BOOT_CMDLINE="console=tty0 console=ttyS0,115200" \
-        "$PROJECT_DIR/iso/build-iso.sh"
+    PROFILE="${PROFILE:-stable-cinnamon}" \
+        "$PROJECT_DIR/iso/build.sh" --profile "${PROFILE:-stable-cinnamon}" --type installer
 else
     blue "reusing cached test ISO $TEST_ISO (set REBUILD_ISO=1 to rebuild)"
 fi