Prebuilt Pi image and a one-line installer

Two routes onto a Pi. The image for a blank card, the script for a board that
already works, which is most of them.

deploy/install.sh installs apt dependencies, clones to /opt/ashvale, builds a
venv with --system-site-packages, enables I2C and installs a systemd unit.
Tested end to end on the real Zero 2 W by installing a second instance on port
8099 alongside the live station: both served, and the live station was
untouched throughout.

That test earned its keep twice. The installer first declared success while the
service was crash-looping on a port clash, because systemd reports active for
the instant between exec and the first failed bind; it now polls the HTTP
endpoint instead, which is the only check that means anything. And my first
attempt to verify that fix was itself worthless, because curl on 127.0.0.1:8000
was answered by the live station rather than the instance under test.

deploy/pi-image is a pi-gen stage on Raspberry Pi OS Lite, Trixie, arm64, which
is exactly what the board runs. Built by .github/workflows/image.yml against a
pinned pi-gen commit, so the artifact does not move when an upstream branch
does, and published to Releases where the 2 GB asset limit comfortably fits a
Lite image.

The image ships no password, no WiFi and no SSH host keys. Baked host keys
would give every person who flashed it the same identity and make them
trivially impersonable on their own network. Coordinates default to Greenwich
at 0 m, wrong for everybody on purpose, because a plausible wrong altitude
quietly biases the sea-level reduction on every row.

The source is copied through a .gitignore filter rather than a hand-written
exclude list, and that is a security property rather than tidiness: the
hand-written list I wrote first missed HANDOVER.md, which is gitignored
precisely because it contains LAN addresses and SSH details. Verified: 69 files,
no state, no local notes, all essentials present.

README.md

@@ -131,6 +131,22 @@ after a fortnight of real data and believe those instead.
 
 ---
 
+## Getting it onto a Pi
+
+Already have a working Raspberry Pi:
+
+```bash
+curl -fsSL https://raw.githubusercontent.com/lynchaos/ashvale-station/main/deploy/install.sh | sudo bash
+```
+
+Blank SD card: download the prebuilt Raspberry Pi OS Lite (Trixie, arm64) image
+from [Releases](https://github.com/lynchaos/ashvale-station/releases), verify the
+checksum, and flash it with Raspberry Pi Imager, setting your own username,
+password and WiFi in the customisation dialog. The image carries no credentials
+and no SSH host keys: those are generated on first boot.
+
+Either way, see [deploy/README.md](deploy/README.md).
+
 ## Install on the Pi
 
 ```bash