Bootstrap DD3 Rust port workspace with host-first compatibility tests

fixtures/contracts/ha_discovery/energy.json

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+{"name":"dd3-F19C Energy","state_topic":"smartmeter/dd3-F19C/state","unique_id":"dd3-F19C_energy","unit_of_measurement":"kWh","device_class":"energy","value_template":"{{ value_json.e_kwh }}","device":{"identifiers":["dd3-F19C"],"name":"dd3-F19C","model":"DD3-LoRa-Bridge","manufacturer":"AcidBurns"}}

fixtures/contracts/mqtt_state/sample.json

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+{"id":"F19C","ts":1769905000,"e_kwh":1234.57,"p_w":322,"p1_w":100,"p2_w":110,"p3_w":111,"bat_v":3.88,"bat_pct":77,"rssi":-71,"snr":7.25,"err_m":1,"err_d":2,"err_tx":3,"err_last":2,"rx_reject":1,"rx_reject_text":"crc_fail"}

fixtures/contracts/sd_csv/sample.csv

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+ts_utc,ts_hms_local,p_w,p1_w,p2_w,p3_w,e_kwh,bat_v,bat_pct,rssi,snr,err_m,err_d,err_tx,err_last
+1769905000,12:34:56,321.6,100.4,110.4,110.8,1234.568,3.88,77,-71,7.3,1,2,3,decode

fixtures/meter/iec_samples/README.md

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+Placeholder fixtures for IEC meter parser phase.
+
+Host-side parser tests will add representative OBIS sample text frames here.

fixtures/protocol/SOURCES.md

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+# Fixture Sources
+
+- Baseline repository: C3MA/DD3-LoRa-Bridge-MultiSender
+- Baseline branch: lora-refactor
+- Baseline commit: a3c61f9b929fbc55bfb502b443fba2f98023b3f1
+- Payload vectors: vendor/dd3-cpp/test/test_payload_codec/test_payload_codec.cpp
+- Payload note: VECTOR_FULL_30 in pinned commit is 183-byte upstream raw (stored as full_30_upstream_raw.bin); canonical full_30.bin appends final two p3 deltas `9A 03 85 03` to satisfy baseline codec semantics.
+- Frame/chunk vectors: derived from vendor/dd3-cpp/test/test_lora_transport/test_lora_transport.cpp semantics