fix(install): reliable stop, VERSION flag, and on-device walkthrough

- aftertouch init script: stop) now waits up to 15 s for SIGTERM
  to take effect, then escalates to SIGKILL; prevents stale daemon
  processes after '/etc/init.d/aftertouch stop' returns (weissigera's
  workaround was manual 'killall aftertouch-service')

- install.sh: add --version / -v CLI flag so the version to install
  can be passed as a command-line argument in addition to the VERSION
  env var; document the trade-off of the hard-coded default in a
  comment; update scripts/on-device-install/README.md with concrete
  usage examples for env-override, CLI flag, and rollback tip

- docs/guides/ON-DEVICE-INSTALL-WALKTHROUGH.md: 10-step runbook
  derived from weissigera's field-tested procedure (issue #329
  comment #4521280831): SSH connection, storage cleanup, install via
  install.sh, reboot, SSH tunnel, Health QuickFix, pairing
  verification, soundtouch-cli download, custom-radio preset setup,
  and final verification; troubleshooting table at the end

Closes #329 (remaining two tasks)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/on-device-install/README.md

@@ -2,6 +2,9 @@
 
 Allows to run AfterTouch on SoundTouch devices directly, eliminating the need to run and maintain a separate server on the local network.
 
+For a complete step-by-step walkthrough — from first SSH connection through verified radio preset playback — see
+[docs/guides/ON-DEVICE-INSTALL-WALKTHROUGH.md](../../docs/guides/ON-DEVICE-INSTALL-WALKTHROUGH.md).
+
 ## Disclaimer
 
 ### Invasiveness
@@ -82,7 +85,31 @@ The init script's `status` now distinguishes "running with listener up" from "PI
 
 ## Updating AfterTouch
 
-To update AfterTouch, simply run the installation command again. The installer will check if there's a new version available and update it if necessary.
+Run the installer again with the version you want to install. The script backs up the currently-running binary (named after its version), installs the new one, and prunes older leftover artefacts to keep `/mnt/nv` free.
+
+**Install (or upgrade to) a specific version** — three equivalent ways:
+
+```bash
+# 1. Environment variable (works when piping into sh)
+VERSION=0.92.0 rw && curl -sSL https://raw.githubusercontent.com/gesellix/Bose-SoundTouch/main/scripts/on-device-install/install.sh | sh
+
+# 2. Command-line flag (pass args after `sh -s --`)
+rw && curl -sSL https://raw.githubusercontent.com/gesellix/Bose-SoundTouch/main/scripts/on-device-install/install.sh | sh -s -- --version 0.92.0
+
+# 3. Download first, then run with a flag
+curl -sSLo install.sh https://raw.githubusercontent.com/gesellix/Bose-SoundTouch/main/scripts/on-device-install/install.sh
+sh install.sh --version 0.92.0
+```
+
+Running **without** a version override installs the version hard-coded in the script (the latest release at the time the script was published). That default is updated with each release; if you're running from `main`, it reflects the most recent tagged version.
+
+> **Tip — rollback:** if the new binary misbehaves, the installer left a `.backup` file alongside it:
+> ```bash
+> ls /mnt/nv/aftertouch/aftertouch-service*.backup
+> cp /mnt/nv/aftertouch/aftertouch-service.<old-version>.backup \
+>    /mnt/nv/aftertouch/aftertouch-service
+> /etc/init.d/aftertouch restart
+> ```
 
 ## Uninstallation
 

scripts/on-device-install/aftertouch

@@ -81,10 +81,24 @@ case "$1" in
   stop)
     echo "Stopping $DESC..."
     if [ -f "$PIDFILE" ]; then
+      PID=$(cat "$PIDFILE")
       start-stop-daemon --stop \
         --quiet \
         --oknodo \
         --pidfile "$PIDFILE"
+      # Wait up to 15 s for SIGTERM to take effect before escalating.
+      # The Go HTTP server exits promptly on SIGTERM in normal conditions;
+      # the loop handles the rare case where it is stuck in a blocking syscall.
+      tries=0
+      while [ $tries -lt 15 ] && kill -0 "$PID" 2>/dev/null; do
+        sleep 1
+        tries=$((tries + 1))
+      done
+      if kill -0 "$PID" 2>/dev/null; then
+        echo "Warning: $NAME (PID $PID) still alive after ${tries}s; sending SIGKILL..." >&2
+        kill -9 "$PID" 2>/dev/null || true
+        sleep 1
+      fi
       rm -f "$PIDFILE"
     else
       echo "No $NAME running (no PID file)." >&2

scripts/on-device-install/install.sh

@@ -1,7 +1,31 @@
 #!/bin/bash
 set -eo pipefail
 
+# Default version installed when no override is provided.  Update this value
+# each time a new release is cut so that running the canonical one-liner
+#   curl -sSL .../install.sh | sh
+# picks up the latest binary without extra arguments.
+#
+# Override via environment variable or the --version/-v flag:
+#   VERSION=0.92.0 curl -sSL .../install.sh | sh
+#   curl -sSL .../install.sh | sh -s -- --version 0.92.0
 VERSION=${VERSION:-0.91.0}
+
+# Parse optional command-line arguments so the script can be invoked as:
+#   install.sh --version 0.92.0
+#   install.sh -v 0.92.0
+while [ $# -gt 0 ]; do
+  case "$1" in
+    --version|-v)
+      if [ -z "$2" ]; then
+        echo "ERROR: --version requires an argument." >&2; exit 1
+      fi
+      VERSION="$2"; shift 2;;
+    --) shift; break;;
+    *) echo "Unknown argument: $1" >&2; exit 1;;
+  esac
+done
+
 GH_REPO=${GH_REPO:-gesellix/Bose-SoundTouch}
 BINARY_URL=${BINARY_URL:-https://github.com/$GH_REPO/releases/download/v$VERSION/soundtouch-service-v$VERSION-linux-armv7}
 INIT_SCRIPT_URL=${INIT_SCRIPT_URL:-https://raw.githubusercontent.com/$GH_REPO/v$VERSION/scripts/on-device-install/aftertouch}