Add Home Assistant dashboard and config for the rack

Minimal HA content to reproduce the Butterfly Rack GUI: a YAML-mode
dashboard (power graphs in mW, temperature graph, dual dBm/mW output
readout, staged seed/booster controls, emergency-off), MQTT template
sensors deriving mW from the dBm telemetry, and an emergency-shutdown
script. Snippets merge into configuration.yaml / scripts.yaml; the
dashboard file is copied whole. See homeassistant/README.md.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

homeassistant/scripts.snippet.yaml

@@ -0,0 +1,19 @@
+# Butterfly EDFA Rack — snippet to merge into Home Assistant's scripts.yaml.
+# Emergency shutdown: ramp pump-1 to 0, disable the booster, then the seed.
+
+butterfly_edfa_shutdown:
+  alias: Butterfly EDFA emergency shutdown
+  icon: mdi:alarm-light
+  mode: single
+  sequence:
+    - service: number.set_value
+      target:
+        entity_id: number.butterfly_edfa_rack_amp_pump_current
+      data:
+        value: 0
+    - service: switch.turn_off
+      target:
+        entity_id: switch.butterfly_edfa_rack_amp_pump
+    - service: switch.turn_off
+      target:
+        entity_id: switch.butterfly_edfa_rack_seed_laser