feat(niri-live): dual-kernel boot menu, audio fix, elogind fix, rsync optimization

- Add kernel 7 (linux-mainline) as primary boot entry
- Add kernel 6 (linux) as secondary fallback boot entry
- Simplified boot menu: single entry per kernel, no failsafe variants
- Add snd-intel-dspcfg.dsp_driver=1 + snd_hda_intel.dmic_detect=0 to BOOT_CMDLINE
- Fix elogind sv: dual D-Bus + PID check to suppress already-running warnings
- Replace cp -rfpPv with rsync -aHX in copy_include_directories (nixpkgs speed fix)
- Fix trailing slash bash glob bug in mklive.sh and postsetup-nvidia.sh
- Add -v linux-mainline to _inner-build-niri-live.sh for correct primary kernel

config/profiles/mainline-niri/packages.live-desktop.list

@@ -6,6 +6,9 @@
 base-system
 linux-mainline
 linux-mainline-headers
+# Kernel 6 (stable) included for dual-boot menu entry fallback.
+linux
+linux-headers
 linux-firmware
 linux-firmware-network
 intel-ucode