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>

.gitignore

@@ -0,0 +1,5 @@
+/target
+*.pkg.tar.zst
+/pkg
+/src/*/
+!/src/*.rs