feat(doorman): badge listener + decoder TDD + deploy scaffolding [#355]

Modern successor of the 2018 Perl doorman (snapshot in legacy/):
bash+coreutils HID decoder, HA webhook dispatch, systemd unit.
Offline decoder suite green (9 tests); shellcheck zero-warning.

Umbrella #345. Work item: https://projects.knownelement.com/issues/355

STATUS.md

@@ -0,0 +1,10 @@
+# STATUS
+
+→ Canonical operations status lives on Discourse:
+  [#297 — Operations Status](https://community.turnsys.com/t/297)
+
+Point-in-time (agent-owned, super sparse):
+
+- Active: **#355** — badge system code & testing (KNEL/doorman repo)
+- Umbrella: #345 physical access control; deployment: #356 (onsite)
+- HA-side work: parallel pfv-bms session (out of this repo's scope)