fix(mqtt,nats): align generated wire protocols with the agreed ApiGear scheme

[w4bremer]

Summary

The generated Rust MQTT and NATS adapters had diverged from the agreed ApiGear wire scheme that the C++17/Qt/Python templates implement, so a Rust client/service could not interoperate with them over the same broker. This aligns both transports with the spec (verified against the C++17 code, not the PDFs — the NATS PDF's swapped property verbs are a known documentation error).

MQTT

• Migrate the adapters to rumqttc's MQTT 5 client. This is required: the peer routes RPC replies via the MQTT 5 ResponseTopic + CorrelationData properties.
• Operations use rpc/<op>; the client now correlates and awaits the reply (previously it published op/<op>/req and silently dropped the response).
• Drop the apigear/ prefix; split the single prop/<p> into set/<p> (change request) and retained prop/<p> (notification); drop the bespoke state topic (retained prop/ carries the state).

NATS

• Drop the apigear. prefix; operations use rpc.<op> (was op.<op>).
• Split prop.<p> into set.<p> (request) and prop.<p> (notification) — this also removes a service-side self-echo.
• Announce service.available and answer the init handshake on init.resp.<clientId> (numeric id) instead of the bespoke state subject.

Core

• The per-interface *Data state structs now serialize with the IDL property names (#[serde(rename = "...")]), so the camelCase wire keys actually deserialize. Previously the snapshot silently fell back to defaults (affected NATS init/state and OLink).

Resulting wire scheme (tb.simple / SimpleInterface)

	MQTT (/)	NATS (.)
operation	…/rpc/<op> (+ MQTT5 ResponseTopic/CorrelationData)	…​.rpc.<op> (request/reply)
property set	…/set/<p>	…​.set.<p>
property notify	…/prop/<p> (retained)	…​.prop.<p>
signal	…/sig/<p>	…​.sig.<p>
availability/init	retained props	…​.service.available + …​.init → …​.init.resp.<id>

Verification

• cargo fmt --check, clippy -D clippy::correctness -D clippy::perf, cargo doc -D warnings: clean
• goldenmaster matches a clean regeneration (go run main.go diff): clean
• 524 unit tests + 46 broker round-trip tests (MQTT/NATS/OLink): green
• Cross-technology interop verified against the real C++17 example binaries over a live broker, in all four directions — Rust client ↔ C++ server and C++ client ↔ Rust server, for both MQTT and NATS. The C++ client reads the Rust server's seeded propInt/propString, and the Rust client's RPC round-trips against the C++ server.

Commits

• fix(core): serialize interface state with IDL property names
• fix(nats): align generated wire protocol with the agreed ApiGear scheme
• fix(mqtt): align generated wire protocol with the agreed ApiGear scheme
• docs: update MQTT/NATS feature docs for the corrected wire scheme