sensord: one reporter for every sensor, replacing the per-sensor scripts

blueline-proximity-lock is 113 lines of shell implementing a subtle contract
(heartbeat inside SOURCE_DOWN_AFTER, seed from the probe banner, report both
edges) that would have been hand-copied for light and again for accel. One
binary, one contract, shipped in the package instead of the rootfs overlay.

packaging/arch/PKGBUILD.prebuilt

@@ -23,8 +23,9 @@ sha256sums=('SKIP' 'SKIP' 'SKIP' 'SKIP' 'SKIP' 'SKIP' 'SKIP' 'SKIP')
 # that arch and omits them otherwise, so package() picks them up conditionally
 # rather than declaring them as sources for every arch.
 if [ "$CARCH" = aarch64 ]; then
-    source+=('souveraine-sessiond-binary' 'souveraine-sessiond.service')
-    sha256sums+=('SKIP' 'SKIP')
+    source+=('souveraine-sessiond-binary' 'souveraine-sessiond.service'
+             'souveraine-sensord-binary' 'souveraine-sensord.service')
+    sha256sums+=('SKIP' 'SKIP' 'SKIP' 'SKIP')
 fi
 
 package() {
@@ -41,6 +42,14 @@ package() {
         install -Dm644 "$srcdir/souveraine-sessiond.service" \
             "$pkgdir/usr/lib/systemd/user/souveraine-sessiond.service"
     fi
+    # Sensor reporter, aarch64 only — the laptop has no iio-sensor-proxy
+    # sources worth reporting and no device state machine listening.
+    if [ -f "$srcdir/souveraine-sensord-binary" ]; then
+        install -Dm755 "$srcdir/souveraine-sensord-binary" \
+            "$pkgdir/usr/bin/souveraine-sensord"
+        install -Dm644 "$srcdir/souveraine-sensord.service" \
+            "$pkgdir/usr/lib/systemd/user/souveraine-sensord.service"
+    fi
     # Secrets rail (user service, owns org.freedesktop.secrets) and machined
     # (system service, machine identity). Both were hand-copied to the phone
     # and owned by no package — same gap as sessiond.

packaging/souveraine-sensord.service

@@ -0,0 +1,21 @@
+[Unit]
+Description=Souveraine sensor reporter (proximity, light, accelerometer)
+# One reporter for every iio-sensor-proxy source. Replaces the per-sensor shell
+# scripts (blueline-proximity-lock and the light/accel copies that were about to
+# be written) — see DEVICE-STATE-MACHINE.md §10 and §12.
+#
+# A user unit, like sessiond: it talks to sessiond over
+# $XDG_RUNTIME_DIR/souveraine/sessiond.sock and needs no privilege at all.
+PartOf=graphical-session.target
+After=souveraine-sessiond.service
+
+[Service]
+ExecStart=/usr/bin/souveraine-sensord
+# The reporter must outlive every transient failure it can hit — a dead sensor
+# stack, a restarting sessiond, monitor-sensor exiting. It handles those in
+# process; this covers the ones it cannot.
+Restart=always
+RestartSec=5
+
+[Install]
+WantedBy=graphical-session.target