fix(install): gzip the rollback backup to reduce on-device disk pressure

Binaries are tens of MB and only growing (Go 1.27 alone added ~655KB
to soundtouch-service via its own new stdlib defaults, unrelated to
this project's code), while the on-device install target (/mnt/nv) is
only tens of MB total. A user already hit "no space left on device"
attempting an update on real hardware.

Stream the pre-update backup straight through gzip instead of cp-then-
gzip: at that point in the script the old binary is still live and the
newly-downloaded one is already sitting in the temp dir, so writing an
intermediate uncompressed backup copy would briefly need three full
binary-sized copies on disk at once. Streaming avoids ever creating
that intermediate copy. Falls back to a plain uncompressed backup if
gzip is unavailable or the stream fails partway, matching prior
behavior exactly.

Both GC loops (pre- and post-install) now also prune stale
*.backup.gz artefacts, and the README's documented rollback command
covers both the compressed and (fallback) uncompressed cases.

Verified locally (not yet on real hardware): streaming path produces
no uncompressed intermediate, the gzip-unavailable fallback still
produces a plain backup, and the documented gunzip+chmod rollback
restores a byte-identical, executable binary.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

scripts/on-device-install/README.md

@@ -177,9 +177,16 @@ redirect to discover the newest tag. If that lookup fails (offline, or a `curl`
 build without `-w` support), it falls back to a pinned version baked into the
 script.
 
-> **Tip — rollback:** if the new binary misbehaves, the installer left a `.backup` file alongside it:
+> **Tip — rollback:** if the new binary misbehaves, the installer left a backup file
+> alongside it, gzip-compressed as `.backup.gz` (plain `.backup`, uncompressed, if
+> `gzip` wasn't available on your device):
 > ```bash
-> ls /mnt/nv/aftertouch/aftertouch-service*.backup
+> ls /mnt/nv/aftertouch/aftertouch-service*.backup*
+> # .backup.gz (compressed):
+> gunzip -c /mnt/nv/aftertouch/aftertouch-service.<old-version>.backup.gz \
+>    > /mnt/nv/aftertouch/aftertouch-service
+> chmod +x /mnt/nv/aftertouch/aftertouch-service
+> # or, for an uncompressed .backup:
 > cp /mnt/nv/aftertouch/aftertouch-service.<old-version>.backup \
 >    /mnt/nv/aftertouch/aftertouch-service
 > /etc/init.d/aftertouch restart

scripts/on-device-install/install.sh

@@ -97,6 +97,7 @@ fi
 # prior run was interrupted.
 echo "Disk usage before pre-install GC:"; df -h "$INSTALL_DIR"
 for f in "$INSTALL_DIR/aftertouch-service".*.backup \
+          "$INSTALL_DIR/aftertouch-service".*.backup.gz \
           "$INSTALL_DIR/aftertouch-service".*.old \
           "$INSTALL_DIR/aftertouch-service.new"; do
   [ -f "$f" ] || continue
@@ -121,8 +122,24 @@ if [ -f "$INSTALL_DIR/aftertouch-service" ]; then
   if [ -z "$current_version" ] || [ "$current_version" = "dev" ]; then
     current_version=$(date +%Y%m%d-%H%M%S)
   fi
+  # Binaries are tens of MB and only growing (see #614 investigation into
+  # Go 1.27's default binary-size increase), while /mnt/nv is small (tens of
+  # MB total). Stream straight into the compressed file rather than cp-then-
+  # gzip: at this point in the script the old binary is still live AND the
+  # newly-downloaded one is already sitting in $UPDATE_TMP_DIR, so an
+  # intermediate uncompressed backup copy would briefly need all three full
+  # copies on disk at once -- exactly the kind of moment that has already
+  # caused "no space left on device" failures here. Best effort: if gzip is
+  # missing, or the stream fails partway (e.g. disk fills mid-compress),
+  # fall back to a plain uncompressed copy exactly as before.
   BACKUP_FILE="$INSTALL_DIR/aftertouch-service.${current_version}.backup"
-  cp -p "$INSTALL_DIR/aftertouch-service" "$BACKUP_FILE"
+  if command -v gzip >/dev/null 2>&1 \
+      && gzip -c < "$INSTALL_DIR/aftertouch-service" > "$BACKUP_FILE.gz"; then
+    BACKUP_FILE="$BACKUP_FILE.gz"
+  else
+    rm -f "$BACKUP_FILE.gz"
+    cp -p "$INSTALL_DIR/aftertouch-service" "$BACKUP_FILE"
+  fi
   echo "Backed up current binary ($current_version) → $BACKUP_FILE"
 fi
 
@@ -136,6 +153,7 @@ chmod +x "$INSTALL_DIR/aftertouch-service"
 if [ -n "$BACKUP_FILE" ]; then
   echo "Disk usage before post-install GC:"; df -h "$INSTALL_DIR"
   for f in "$INSTALL_DIR/aftertouch-service".*.backup \
+            "$INSTALL_DIR/aftertouch-service".*.backup.gz \
             "$INSTALL_DIR/aftertouch-service".*.old \
             "$INSTALL_DIR/aftertouch-service.new"; do
     [ -f "$f" ] || continue