Add runtime status checker + headless/X11 docs; distro-support refinements

- status.sh: runtime health check (service state, boot wiring, display backend auto-detect, encoder, ports, web UI, /dev/uinput, pairing) ending in a g2g verdict or concrete TODO list

- docs: TROUBLESHOOTING §12 (headless graphical-session.target boot trap) + §13 (X11/NVENC path & stale wlr drop-in env conflict); ARCHITECTURE capture-backend comparison; FOLLOWUPS P3 (installer X11/Ubuntu support); README diagnostics pointer

- lib/{config,packages,permissions,service}.sh, files/sway-headless.service: in-progress Debian/Ubuntu + headless support refinements

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/packages.sh

@@ -108,11 +108,11 @@ _ubuntu_sunshine_deb_url() {
 }
 
 _install_sunshine_debian() {
-  # Universal runtime deps. libva-utils gives `vainfo`; jq is used by hooks.
+  # Universal runtime deps. vainfo is what Ubuntu calls libva-utils on Arch.
   # pipewire-pulse is the Ubuntu 24.04+ default audio path; on older releases
   # `pulseaudio-utils` works too — we don't force the codename split since
   # sunshine just needs *a* PulseAudio API endpoint.
-  pkg_install jq vulkan-tools libva-utils curl ca-certificates
+  pkg_install jq vulkan-tools vainfo curl ca-certificates
 
   if pkg_installed sunshine; then
     ok "sunshine already installed (dpkg)"
@@ -198,15 +198,36 @@ install_gpu_encoder_packages() {
       yay_install intel-media-driver vulkan-intel
       ;;
     debian:nvidia)
-      # On Ubuntu, the proprietary driver is usually already installed via
-      # `ubuntu-drivers autoinstall` or the Server install path. Don't force a
-      # specific nvidia-* version — they vary by release / driver branch.
-      # Pull only the userspace VAAPI bridge if available; harmless if missing.
-      pkg_install libnvidia-encode-no-dkms 2>/dev/null \
-        || pkg_install libnvidia-encode-575 2>/dev/null \
-        || pkg_install libnvidia-encode-565 2>/dev/null \
-        || pkg_install libnvidia-encode-560 2>/dev/null \
-        || info "NVENC userspace library not found via a known package name — relying on the existing driver install."
+      # Ubuntu's nvidia-driver-NNN(-server) metapackage pulls in the matching
+      # libnvidia-encode-NNN(-server) as a dependency, so NVENC is normally
+      # already present. Only intervene if it's missing — and derive the right
+      # package name from the loaded driver's major version instead of
+      # guessing.
+      if dpkg-query -W -f='${Status}\n' 'libnvidia-encode-*' 2>/dev/null \
+         | grep -q '^install ok installed$'; then
+        ok "NVENC runtime library already installed via the driver metapackage"
+      else
+        local drv_major drv_full
+        drv_full="$(nvidia-smi --query-gpu=driver_version --format=csv,noheader,nounits 2>/dev/null | head -n1)"
+        drv_major="${drv_full%%.*}"
+        if [[ -z "$drv_major" ]]; then
+          warn "Could not detect NVIDIA driver version; NVENC may be missing."
+        else
+          # Try -server first (cloud GPUs usually run server branches), then the
+          # consumer branch. Stop at the first one apt knows about.
+          local picked=""
+          for cand in "libnvidia-encode-${drv_major}-server" "libnvidia-encode-${drv_major}"; do
+            if apt-cache show "$cand" >/dev/null 2>&1; then
+              picked="$cand"; break
+            fi
+          done
+          if [[ -n "$picked" ]]; then
+            pkg_install "$picked"
+          else
+            warn "No libnvidia-encode-${drv_major}* package in apt — install it manually if NVENC fails."
+          fi
+        fi
+      fi
       ;;
     debian:amd)
       pkg_install mesa-va-drivers mesa-vulkan-drivers vainfo