rewrite as a daemon with shutdown on low battery

the oneshot+timer produced 8367 journal lines a day of systemd
start/stop noise. now Type=notify with an internal ticker, logging
transitions only.

hardware sits behind sensor.Sensor; internal/monitor is pure, so the
shutdown state machine is tested without a pi.

four guards, each tested, because a false poweroff of a box with no
physical access is worse than a missed one:
- a failed read resets the arming window rather than pausing it
- voltage must actually fall across the window, else it is a stuck
  sensor. this is the only defence against the ac line reading 0
  while mains is connected, so it is load-bearing
- a settle period stops a restart loop acting on early readings
- flapping ac cannot accumulate

verified on hardware 2026-08-09: after poweroff on battery the x1208
starts the pi again when mains returns, with no button press. that
was the risk that could have made this feature unsafe.

a failed tick rewrites the previous sample with sensor_healthy 0 and
the failure counter, so last_update still freezes for the staleness
alert while the failure stays visible.

module path moved to git.mial.net. debian units and the compose
overlay deleted; deployment lives in the infra repo.

justfile

@@ -1,23 +1,40 @@
+# x1208-exporter — Geekworm X1208 UPS HAT exporter
+#
+# Deployment lives in the rpi.mial.net repo. There are no deploy recipes here.
+
 default:
     @just --list
 
+# Compile everything
 build:
-    GOOS=linux GOARCH=arm64 go build -ldflags="-s -w" -o build/x1208-exporter ./...
+    go build ./...
 
-deploy: build
-    scp build/x1208-exporter rpi:/tmp/x1208-exporter
-    scp systemd/x1208-exporter.service systemd/x1208-exporter.timer rpi:/tmp/
-    ssh rpi 'sudo install -m 0755 /tmp/x1208-exporter /usr/local/bin/x1208-exporter && \
-        sudo install -m 0644 /tmp/x1208-exporter.service /etc/systemd/system/ && \
-        sudo install -m 0644 /tmp/x1208-exporter.timer /etc/systemd/system/ && \
-        sudo install -d -m 0755 -o root /var/lib/node-exporter/textfiles && \
-        sudo systemctl daemon-reload && \
-        sudo systemctl enable --now x1208-exporter.timer && \
-        sudo systemctl start x1208-exporter.service && \
-        cat /var/lib/node-exporter/textfiles/x1208.prom'
+# Run the test suite (all pure packages; the hardware packages need a Pi)
+test:
+    go test ./...
 
-logs:
-    ssh rpi 'journalctl -u x1208-exporter.service -n 30 --no-pager'
+test-verbose:
+    go test -v ./...
 
-run-once:
-    ssh rpi 'sudo systemctl start x1208-exporter.service && cat /var/lib/node-exporter/textfiles/x1208.prom'
+# Static checks
+lint:
+    go vet ./...
+    gofmt -l .
+
+fmt:
+    gofmt -w .
+
+# Build the Nix package exactly as the Pi will
+nix-build:
+    nix build .#x1208-exporter
+
+# Run the daemon against a scripted sensor. Touches no hardware and powers
+# nothing off — the shutdown command is replaced with an echo.
+run-fake out="/tmp/x1208.prom":
+    go run . -fake -interval 1s -out {{ out }} \
+        -shutdown -shutdown-settle-ticks 2 -shutdown-samples 3 \
+        -shutdown-cmd "/usr/bin/env echo POWEROFF-WOULD-RUN"
+
+# Print the metrics the fake run produced
+show out="/tmp/x1208.prom":
+    @cat {{ out }}