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/).

Makefile

@@ -1,8 +1,12 @@
 # Void Installer — XPS 17 (xps9700)
 #
+# Profiles: stable-cinnamon | stable-niri | mainline-cinnamon | mainline-niri
+#   kernel: stable = Void's linux (k6),  mainline = linux-mainline (k7)
+#   DE:     cinnamon = Cinnamon/X11,     niri = niri+noctalia-shell/Wayland
+#
 # Targets:
-#   make iso         build the auto-installing ISO (uses docker)
-#   make live        build the full Cinnamon live desktop ISO
+#   make iso         build the auto-installing ISO   (PROFILE=stable-cinnamon)
+#   make live        build the full desktop live ISO  (PROFILE=stable-cinnamon)
 #   make test-disk   create a fresh QEMU test disk that mimics XPS 17 layout
 #   make test        full automated QEMU smoke test
 #   make test-iso    rebuild only the TEST ISO variant
@@ -10,10 +14,16 @@
 #   make shellcheck  lint all installer/build shell scripts
 #   make clean       remove build/, out/  (cache stays)
 #   make distclean   also remove cache/
+#
+# Examples:
+#   make live PROFILE=mainline-niri
+#   make iso  PROFILE=stable-cinnamon
+#   make test PROFILE=mainline-cinnamon
 
 PROJECT_DIR := $(CURDIR)
 OUT         := $(PROJECT_DIR)/out
 SECRETS     := $(PROJECT_DIR)/secrets.env
+PROFILE     ?= stable-cinnamon
 
 .PHONY: all iso live live-qemu test test-disk test-iso qemu shellcheck clean distclean check-secrets check-docker
 
@@ -27,10 +37,10 @@ check-docker:
 	@docker info >/dev/null 2>&1 || { echo "ERROR: docker daemon unreachable (in 'docker' group? systemctl start docker?)"; exit 1; }
 
 iso: check-secrets check-docker
-	$(PROJECT_DIR)/iso/build-iso.sh
+	$(PROJECT_DIR)/iso/build.sh --profile $(PROFILE) --type installer
 
 live: check-secrets check-docker
-	$(PROJECT_DIR)/iso/build-live-iso.sh
+	$(PROJECT_DIR)/iso/build.sh --profile $(PROFILE) --type live
 
 # Launch the live ISO in QEMU with 12 GB RAM so nix packages fit in the tmpfs.
 # The live session is a pure-RAM tmpfs overlay; Cinnamon + nix need ~7-8 GB total.
@@ -38,14 +48,14 @@ live-qemu:
 	$(PROJECT_DIR)/tests/launch-live-qemu.sh
 
 test-iso: check-secrets check-docker
-	REBUILD_ISO=1 $(PROJECT_DIR)/tests/run-qemu-test.sh
+	REBUILD_ISO=1 PROFILE=$(PROFILE) $(PROJECT_DIR)/tests/run-qemu-test.sh
 
 test-disk:
 	$(PROJECT_DIR)/tests/make-test-disk.sh $(OUT)/test-disk.img
 
 test: check-secrets check-docker
 	@mkdir -p $(OUT)
-	$(PROJECT_DIR)/tests/run-qemu-test.sh
+	PROFILE=$(PROFILE) $(PROJECT_DIR)/tests/run-qemu-test.sh
 
 qemu:
 	$(PROJECT_DIR)/tests/interactive-qemu.sh
@@ -55,10 +65,8 @@ shellcheck:
 	shellcheck -x \
 	    $(PROJECT_DIR)/installer/install.sh \
 	    $(PROJECT_DIR)/installer/lib/*.sh \
-	    $(PROJECT_DIR)/iso/build-iso.sh \
-	    $(PROJECT_DIR)/iso/_inner-build.sh \
-	    $(PROJECT_DIR)/iso/_inner-build-live.sh \
-	    $(PROJECT_DIR)/iso/build-live-iso.sh \
+	    $(PROJECT_DIR)/iso/build.sh \
+	    $(PROJECT_DIR)/iso/_inner-build-unified.sh \
 	    $(PROJECT_DIR)/tests/*.sh \
 	    $(PROJECT_DIR)/tests/lib/*.sh