Bootstrap DD3 Rust port workspace with host-first compatibility tests

docs/INTEROP_CHECKLIST.md

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+# Interop Checklist
+
+## A) Rust Receiver <-> C++ Sender
+1. Run C++ sender on `lora-refactor` baseline and Rust receiver host simulation harness.
+2. Verify receiver accepts sender short-id and sender-id mapping.
+3. Verify ACK payload bytes:
+- `[flags:1][batch_id_be:2][epoch_utc_be:4]`
+4. Verify duplicate batch handling:
+- ACK still sent
+- duplicate counters increment
+- publish/log suppressed
+5. Compare MQTT state and fault payload keys/semantics against baseline.
+6. Compare CSV line/header output for shared sample windows.
+
+## B) Rust Sender <-> C++ Receiver
+1. Run Rust sender with sync-request startup behavior.
+2. Confirm C++ receiver decodes payload schema v3 and sparse mask reconstruction.
+3. Confirm ACK mismatch handling does not clear inflight batch.
+4. Confirm retry and catch-up behavior under injected packet loss.
+
+## C) Contract Comparison
+1. Home Assistant discovery topics and payload fields:
+- topic: `homeassistant/sensor/<device_id>/<key>/config`
+- `unique_id`, `device.identifiers`, `device.name`, `device.model`, `device.manufacturer`
+2. MQTT state payload:
+- required keys present
+- legacy keys absent
+3. CSV output:
+- exact header order
+- stable numeric formatting
+
+## D) Port Validation Items
+- Sender unsynced boot sends only sync-requests.
+- ACK bootstrap gate enforced (`time_valid=1` and epoch >= `MIN_ACCEPTED_EPOCH_UTC`).
+- Frame/CRC/ACK/payload golden fixtures all pass.
+- All protocol encode/decode paths covered by fixture-backed tests.