presence: BLE proximity events for a kiosk screen

A swayidle-shaped daemon: `present <cmd>` runs when a Bluetooth LE device
comes within an RSSI threshold, `absent <secs> <cmd>` once nobody has for
that long. Near means 3 of the last 5 advertisements at or above -58 dBm by
default; --calibrate prints the raw stream so the threshold is picked at the
installed screen, not guessed. Nothing is stored: a few seconds of samples in
memory, counted.

Packaged for the [uhhm] registry from the checkout itself, built natively per
arch on tag by Gitea Actions, with the version checked across Cargo.toml,
PKGBUILD and the tag.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

PKGBUILD

@@ -0,0 +1,31 @@
+# Maintainer: bl <bendik@lynghaug.me>
+# Built from the checkout it sits in: .gitea/workflows/release.yml runs makepkg
+# at the repo root on a v* tag, natively per arch, and uploads to the [uhhm]
+# registry. pkgver must match Cargo.toml and the tag; CI checks all three.
+#
+# source=() and $startdir on purpose: makepkg's own $srcdir is ./src, which is
+# where the Rust sources live, so copying into it would nest src/src. Cargo
+# builds in place into ./target (gitignored) instead.
+
+pkgname=presence
+pkgver=0.1.0
+pkgrel=1
+pkgdesc="BLE proximity events for a kiosk screen, swayidle style"
+arch=('x86_64' 'aarch64')
+url="https://project.uhhm.no/uhhm/presence"
+license=('MIT')
+depends=('bluez' 'dbus' 'gcc-libs' 'glibc')
+makedepends=('cargo')
+source=()
+
+build() {
+    cd "$startdir"
+    CARGO_TARGET_DIR="$startdir/target" cargo build --release --locked
+}
+
+package() {
+    cd "$startdir"
+    install -Dm755 target/release/presence "$pkgdir/usr/bin/presence"
+    install -Dm644 README.md "$pkgdir/usr/share/doc/presence/README.md"
+    install -Dm644 LICENSE "$pkgdir/usr/share/licenses/presence/LICENSE"
+}