chore(adr): record protobuf rpc over nats micro binding decision

[yordis]

• First-party protobuf services on NATS had no governing rule for subjects, error signaling, or discovery, exposing them to the same mapping drift that motivated ADR 0011 for the JSON-RPC family.
• The trogon.nats.micro.v1alpha1 annotations already ship in trogon-proto without an ADR defining what they mean on the wire.

[cursor]

PR Summary

Low Risk
Documentation-only; no runtime or implementation changes in this PR.

Overview
Adds ADR 0016, which records how first-party protobuf service definitions map onto NATS micro (the protobuf counterpart to ADR 0011 for JSON-RPC).

The decision ties trogon.nats.micro.v1alpha1 annotations to wire behavior: each annotated service becomes one micro service with per-rpc endpoints, subjects derived from prefix + service + method name, success vs failure signaled only via Nats-Service-Error-Code, content-type rules from ServiceOptions, and shared discovery/load-balancing via micro rather than per-service inventing. It explicitly rejects JSON-RPC binding, ad-hoc mappings, and tunneled gRPC.

The ADR index gains a link to the new record. ADR 0017 (decider commands) is referenced as a specialization of this binding.

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[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

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📥 Commits

Reviewing files that changed from the base of the PR and between d840eb8 and 571e4cc.

📒 Files selected for processing (2)

• docs/adr/0016-protobuf-rpc-over-nats-micro-binding.md
• docs/adr/index.md

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Walkthrough

This PR adds a new Architecture Decision Record (ADR 0016) documenting how annotated Protocol Buffers services bind to NATS micro, covering service/endpoint mapping, subject derivation, error discrimination via headers, content-type negotiation, and lists it in the ADR index.

Changes

ADR Documentation

Layer / File(s)	Summary
ADR 0016 content docs/adr/0016-protobuf-rpc-over-nats-micro-binding.md	New ADR defining protobuf service-to-NATS-micro-service and rpc-to-endpoint mapping, subject derivation, success/error discrimination via the Nats-Service-Error-Code header, content-type negotiation, invariants, rejected alternatives, consequences, and references.
ADR index registration docs/adr/index.md	Adds an entry linking to ADR 0016 in the Records index list.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Possibly related PRs

• TrogonStack/trogonai#396: Both PRs add ADR documentation for NATS-based RPC bindings and update docs/adr/index.md to register the new ADR entries.

Poem

A rabbit hops through docs so neat,
ADR sixteen, quite a feat! 🐇
Protobuf and NATS now bind as one,
Subjects derived, the mapping's done,
Headers whisper success or strife,
Index updated, docs come to life. 📜✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the new ADR recording the protobuf RPC over NATS micro binding decision.
Description check	✅ Passed	The description matches the PR goal by discussing subject mapping, error signaling, discovery, and the existing NATS micro annotations.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch yordis/copy-adr-0016-nats-rpc

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