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/_inner-build-niri-live.sh

@@ -208,6 +208,10 @@ trap _cleanup_mklive_builds EXIT
 
 # mklive prepends -r args: LAST -r = HIGHEST priority.
 # Our local signed repo is last so xbps resolves noctalia-* from it.
+# -v linux-mainline: makes mainline (kernel 7) the PRIMARY boot kernel;
+#   mklive.sh sets KERNELVERSION from it and ignores the linux metapackage.
+#   The plain 'linux' (kernel 6) in ISO_PKGS still gets installed and the
+#   secondary-kernel loop in mklive.sh builds its initramfs + boot entry.
 ./mklive.sh \
     -a "$ARCH" \
     -r "$REPO_URL" \
@@ -219,6 +223,7 @@ trap _cleanup_mklive_builds EXIT
     -k "$KEYMAP" \
     -l "$LOCALE" \
     -T "$ISO_TITLE" \
+    -v linux-mainline \
     -p "$ISO_PKGS" \
     -I "$INCLUDE_DIR" \
     -x "$PROJECT_DIR/iso/postsetup-nvidia.sh" \