feat: add live-qemu target with 12GB RAM for nix package headroom

Live session is a pure-RAM tmpfs overlay. Cinnamon takes ~2.5GB, nix
packages take ~4GB — 6GB QEMU was too tight. 12GB gives comfortable
headroom. Real XPS 9700 (32GB) already works fine.

- tests/launch-live-qemu.sh: dedicated QEMU launcher, RAM_MB=12288
- Makefile: 'make live-qemu' target

tests/launch-live-qemu.sh

@@ -0,0 +1,59 @@
+#!/bin/bash
+# Launch the live desktop ISO under QEMU for interactive testing.
+# Uses 12 GB RAM so nix packages (~4 GB) can install into the tmpfs overlay.
+#
+# Usage:
+#   bash tests/launch-live-qemu.sh                  # uses out/void-live-stable.iso
+#   ISO=out/my.iso bash tests/launch-live-qemu.sh
+#   RAM_MB=16384 bash tests/launch-live-qemu.sh     # override RAM
+
+set -Eeuo pipefail
+
+PROJECT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+OUT_DIR="$PROJECT_DIR/out"
+
+ISO="${ISO:-$(ls -t "$OUT_DIR"/void-live-stable*.iso 2>/dev/null | head -1 || true)}"
+[[ -r "$ISO" ]] || { echo "no ISO found — run 'make live' first"; exit 1; }
+
+RAM_MB="${RAM_MB:-12288}"   # 12 GB: Cinnamon ~2.5GB + nix store ~4GB + headroom
+SMP="${SMP:-4}"
+
+OVMF_CODE="${OVMF_CODE:-}"
+OVMF_VARS_TPL="${OVMF_VARS_TPL:-}"
+for c in /usr/share/OVMF/OVMF_CODE.fd /usr/share/edk2-ovmf/x64/OVMF_CODE.fd \
+         /usr/share/edk2/x64/OVMF_CODE.fd /usr/share/qemu/OVMF_CODE.fd; do
+    [[ -z "$OVMF_CODE" && -r "$c" ]] && OVMF_CODE="$c"
+done
+for v in /usr/share/OVMF/OVMF_VARS.fd /usr/share/edk2-ovmf/x64/OVMF_VARS.fd \
+         /usr/share/edk2/x64/OVMF_VARS.fd /usr/share/qemu/OVMF_VARS.fd; do
+    [[ -z "$OVMF_VARS_TPL" && -r "$v" ]] && OVMF_VARS_TPL="$v"
+done
+[[ -r "$OVMF_CODE"     ]] || { echo "OVMF_CODE not found — install ovmf"; exit 1; }
+[[ -r "$OVMF_VARS_TPL" ]] || { echo "OVMF_VARS not found — install ovmf"; exit 1; }
+
+VARS="$OUT_DIR/OVMF_VARS.live.fd"
+[[ -f "$VARS" ]] || cp "$OVMF_VARS_TPL" "$VARS"
+
+mkdir -p "$OUT_DIR"
+
+echo ">>> launching live ISO: $ISO"
+echo "    RAM: ${RAM_MB} MB  CPUs: $SMP"
+echo "    serial: $OUT_DIR/live-serial.sock"
+echo "    monitor: $OUT_DIR/qemu-monitor.sock"
+
+exec qemu-system-x86_64 \
+    -name void-live-test \
+    -machine q35,accel=kvm:tcg \
+    -cpu max \
+    -m "$RAM_MB" \
+    -smp "$SMP" \
+    -drive "if=pflash,format=raw,readonly=on,file=$OVMF_CODE" \
+    -drive "if=pflash,format=raw,file=$VARS" \
+    -cdrom "$ISO" \
+    -boot order=d,menu=off \
+    -netdev user,id=n0 \
+    -device virtio-net-pci,netdev=n0 \
+    -serial "unix:$OUT_DIR/live-serial.sock,server,nowait" \
+    -monitor "unix:$OUT_DIR/qemu-monitor.sock,server,nowait" \
+    -device virtio-vga \
+    -display gtk,gl=off