[discostur]

Closes the README item "call external scripts".

--exec /path/to/script runs a program once per reading. The reading is handed over twice so a script can use whichever is easier — as MITEMPR_* environment variables, and as one JSON object on stdin:

#!/bin/sh
[ -n "$MITEMPR_TEMPERATURE" ] || exit 0
echo "$MITEMPR_NAME is at ${MITEMPR_TEMPERATURE} C"

Every variable is always set, empty when the sensor did not report that measurement. Omitting them instead would let a value inherited from mitempr's own environment show through, and an empty string is easy to test for in shell.

Rate limiting, because a Pi Zero W has one core and sensors advertise every second or two:

• --exec-interval SECS is the shortest gap between two runs for the same sensor, tracked per address. Default 0, i.e. every reading.
• At most four hooks run at once. Beyond that, readings are dropped with a warning rather than queued: a backlog of stale temperatures is worse than a gap, and an unbounded queue on this hardware ends badly.
• A hook that has not finished within 30 seconds is killed, so a stuck script cannot hold one of those four slots forever.

The child is spawned into its own task, so a slow script delays its own next run but never the scan.

Its stdout goes to /dev/null on purpose: with --format json, anything the script printed would otherwise interleave with the reading stream and corrupt it. stderr is inherited so a broken script can still complain visibly.

Five tests, including one end-to-end case that writes a real shell script to a temp directory and checks what it actually received on stdin and in its environment.

---

I went through the README TODO list and found a few real bugs on the way, so this
is one of ten PRs. They are meant to be reviewed and merged in this order, and
each is a separate branch so you can take some and leave others:

1. chore/ci-deps-and-metadata — CI, unused deps, lockfile
2. test/decoder-assertions — make the decoder tests actually assert
3. fix/bthome-object-lengths — BTHome parser desync + encryption flag
4. fix/mibeacon-frame-control — MiBeacon hardcoded offset + encryption flag
5. fix/continuous-scan — one reading per sensor ever, and the watchdog loop
6. feat/logging-and-output (draft) — log levels, --format json
7. feat/config-file (draft) — name, calibrate and filter sensors
8. feat/exec-hook (draft) — --exec
9. feat/prometheus (draft) — --metrics-addr and --pushgateway-url
10. feat/encrypted-payloads (draft) — AES-CCM bind keys

6–10 are drafts because they build on each other; GitHub can only point a
cross-fork PR at main, so until the ones before it are merged each diff also
shows its ancestors. I will rebase and force-push as things merge, so each
shrinks to its own change. Nothing here is urgent — take them at whatever pace
suits, or tell me to close some.

Everything is checked with cargo fmt --check, cargo clippy -D warnings and
cargo test in a rust:1.96 container, since bluer's bluetoothd feature is
Linux-only and I develop on a Mac.