examples: add utility-meter publisher

Reference implementation of an eBus utility-meter publisher
(`energy.ebus.device.utility-meter`, per the Electrification Bus
specification's `data-models/utility-meter.md`). Single-file script
following the same `*Device` / `*DeviceAdapter` shape as `simple-device`:

- `UtilityMeter` — pure-Python model holding values for the four
  standard capabilities (`info`, `meter`, `status`, `doe`).
- `UtilityMeterAdapter` — translates the model to a Homie 5 device
  with capability-typed nodes and property datatype/unit hints.
- DOE HTTP endpoint — `POST /doe/import-limit` accepting
  `{watts, source, validUntil}` to set the dynamic operating envelope
  at runtime.
- Periodic `tick()` stub for synthetic measurement updates.

Includes `utility-meter-cfg.example.json` with a generic nameplate
and an example `meter` sample. Broker config is supplied separately
in the same format as the other examples.

Suitable for a Raspberry Pi or any Linux host as a stand-in utility
meter on a test bench.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dcj

examples/README.md

@@ -20,6 +20,22 @@ Controller that auto-discovers Homie devices and monitors property changes.
 ./simple-controller --config /path/to/broker-cfg.json
 ```
 
+### utility-meter
+
+Publishes a single eBus utility-meter device (`energy.ebus.device.utility-meter`, per the Electrification Bus `data-models/utility-meter.md`) with the standard `info` / `meter` / `status` / `doe` capabilities. Includes a local HTTP endpoint for runtime DOE updates (`POST /doe/import-limit`).
+
+```bash
+./utility-meter --config ./utility-meter-cfg.example.json --broker-config /path/to/broker-cfg.json
+```
+
+Set DOE values at runtime:
+
+```bash
+curl -X POST http://localhost:8765/doe/import-limit \
+  -H 'Content-Type: application/json' \
+  -d '{"watts": 12000, "source": "GRID", "validUntil": "2026-06-07T19:00:00Z"}'
+```
+
 ### simple-span-controller
 
 SPAN Panel controller with mDNS discovery. Connects via MQTTS and monitors power flow properties.