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

iso/postsetup-nvidia.sh

@@ -53,7 +53,8 @@ sed 's/^/    /' "$ROOTFS/etc/dracut.conf.d/10-nvidia.conf"
 found=0
 for kdir in "$ROOTFS/usr/lib/modules"/*/; do
     [[ -d "${kdir}/kernel" ]] || continue
-    kver="${kdir##*/}"
+    # Strip trailing slash before extracting basename (glob */ appends /)
+    kver="${kdir%/}"; kver="${kver##*/}"
     found=1
 
     # Verify nvidia.ko is present for this kernel version.