docs: add post-strict full go parity design

docs/plans/2026-02-23-nats-post-strict-full-go-parity-design.md

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+# NATS Post-Strict Full Go Parity Design
+
+**Date:** 2026-02-23  
+**Status:** Approved  
+**Scope:** Identify and close all remaining NATS functionality/features versus local Go HEAD (`golang/nats-server/server`) using behavior-level parity with test evidence.
+
+## 1. Architecture and Scope Boundary
+
+### Parity model
+Each capability is considered complete only when all of the following are true:
+
+1. Behavior  
+Observable .NET runtime behavior matches the Go contract.
+
+2. Tests  
+Positive and edge/failure tests exist and fail on regression.
+
+3. Docs  
+`differences.md` and parity artifacts reflect verified behavior state.
+
+### Scope rule
+This cycle is not constrained by existing parity-table status values. Residual behavior differences inferred from code depth, runtime paths, and subsystem contracts are in scope even when rows are currently marked `Y`.
+
+### Execution waves
+1. Protocol/inter-server/account delivery semantics  
+2. MQTT full protocol/runtime semantics  
+3. JetStream runtime/state-machine semantics  
+4. RAFT/cluster governance + storage durability semantics  
+5. Operational/config/runtime drift closure and docs synchronization
+
+### Completion gate
+Done is defined as feature-by-feature .NET behavioral parity evidence (targeted + full suite), not mandatory direct Go suite equivalence.
+
+## 2. Component Plan and Remaining Capability Groups
+
+### A. Core protocol and inter-server semantics
+Primary files:
+- `src/NATS.Server/NatsServer.cs`
+- `src/NATS.Server/Routes/*`
+- `src/NATS.Server/Gateways/*`
+- `src/NATS.Server/LeafNodes/*`
+- `src/NATS.Server/Subscriptions/SubList.cs`
+
+Remaining group:
+- deep account-aware forwarding semantics
+- advanced interest/topology behavior beyond baseline frame propagation
+
+### B. MQTT full parity
+Primary files:
+- `src/NATS.Server/Mqtt/*`
+- `src/NATS.Server/MqttOptions.cs`
+- integration points in auth/config/server startup paths
+
+Remaining group:
+- behavior depth still below Go MQTT subsystem semantics despite packet/QoS/session/auth baseline
+- requires broader protocol/runtime contract coverage
+
+### C. JetStream runtime/service depth
+Primary files:
+- `src/NATS.Server/JetStream/JetStreamService.cs`
+- `src/NATS.Server/JetStream/StreamManager.cs`
+- `src/NATS.Server/JetStream/ConsumerManager.cs`
+- `src/NATS.Server/JetStream/Consumers/*`
+- `src/NATS.Server/JetStream/MirrorSource/*`
+
+Remaining group:
+- simplified manager/service behavior relative to Go stream/consumer runtime depth
+- needs stricter long-run invariants and edge-state handling
+
+### D. RAFT, JetStream governance, and storage
+Primary files:
+- `src/NATS.Server/Raft/*`
+- `src/NATS.Server/JetStream/Cluster/*`
+- `src/NATS.Server/JetStream/Storage/*`
+
+Remaining group:
+- broader convergence/placement/membership semantics
+- deeper durability and consistency guarantees under restart/failure scenarios
+
+### E. Operational/config/runtime parity
+Primary files:
+- `src/NATS.Server/NatsOptions.cs`
+- `src/NATS.Server/Configuration/*`
+- `src/NATS.Server/Monitoring/*`
+- `src/NATS.Server/Events/*`
+
+Remaining group:
+- config surface vs runtime behavior depth mismatches
+- monitoring/operations parity consistency closure
+
+### F. Evidence/documentation
+Primary files:
+- `differences.md`
+- `docs/plans/2026-02-23-nats-strict-full-go-parity-map.md`
+- `docs/plans/2026-02-23-jetstream-remaining-parity-map.md`
+- `docs/plans/2026-02-23-jetstream-remaining-parity-verification.md`
+
+Remaining group:
+- synchronize claims with measurable behavior and test evidence
+
+## 3. Data Flow and Behavioral Contracts
+
+1. Inter-server routing contract  
+- account-scoped interest and delivery must remain correct under churn/reconnect/replay
+- transport markers/remaps remain internal and reversible
+
+2. MQTT protocol/runtime contract  
+- packet framing/encoding correctness and session/QoS/ack state-machine consistency
+- auth/tls/account mapping fail closed on invalid state
+
+3. JetStream stream/consumer contract  
+- retention mode semantics diverge at runtime behavior
+- deterministic cursor/ack/redelivery/backoff/replay/flow transitions
+- mirror/source behavior respects account/filter/transform contracts
+
+4. RAFT/governance/storage contract  
+- quorum-gated commit visibility, term safety, and convergence
+- snapshot/membership/placement semantics reflect committed state
+- storage invariants hold across prune/recovery/restart paths
+
+5. Documentation contract  
+- no closure claim without behavior+test evidence
+- deferred items must be explicit and evidence-backed
+
+## 4. Error Handling, Testing Strategy, and Completion Gates
+
+### Error handling
+1. Preserve protocol-specific error semantics (NATS, MQTT, JetStream).  
+2. Fail closed on account/auth/consensus violations.  
+3. Avoid partial state mutation on multi-step cross-subsystem operations.
+
+### Testing strategy
+1. Test-first for every capability.  
+2. Require happy-path + edge/failure coverage per capability.  
+3. Use integration tests for inter-server, MQTT runtime, RAFT/cluster, and restart durability semantics.  
+4. Use focused unit tests for deterministic parser/state primitives.
+
+### Completion gates
+1. Capability closes only when Behavior + Tests + Docs are complete.  
+2. Focused wave suites pass and full suite passes:
+- `dotnet test -v minimal`
+3. Documentation artifacts are synchronized to verified runtime behavior.