docs: add strict full go parity design

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

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+# NATS Strict Full Go Parity Design
+
+**Date:** 2026-02-23  
+**Status:** Approved  
+**Scope:** Identify and close all remaining NATS functionality gaps between Go and .NET using behavior-level parity (not limited to existing parity-table status), with required unit/integration test coverage and parity-doc synchronization.
+
+## 1. Architecture and Scope Boundary
+
+### Closure model
+Each parity capability closes only when all three are complete:
+
+1. Behavior  
+The .NET runtime behavior matches Go contract semantics at protocol and subsystem level.
+
+2. Tests  
+Positive and edge/failure tests exist and fail on regression.
+
+3. Docs  
+`differences.md` and parity artifacts reflect the validated behavior state.
+
+### Scope interpretation
+This cycle treats remaining parity as capability-level and code-backed, not as a direct read of current table status alone. Existing contradictions between prose claims and observable implementation are treated as parity debt and are in scope.
+
+### Execution order
+1. Inter-server fabric semantics (routes/gateway/leaf, account-scoped interest/delivery)  
+2. Auth/account/config/monitor operational semantics  
+3. JetStream runtime state machines  
+4. JetStream storage durability + RAFT/cluster governance  
+5. MQTT protocol/transport parity beyond current baseline  
+6. Documentation and evidence synchronization
+
+## 2. Component Plan
+
+### A. Inter-server fabric parity
+Primary files:
+- `src/NATS.Server/NatsServer.cs`
+- `src/NATS.Server/Routes/RouteConnection.cs`
+- `src/NATS.Server/Routes/RouteManager.cs`
+- `src/NATS.Server/Gateways/GatewayConnection.cs`
+- `src/NATS.Server/Gateways/GatewayManager.cs`
+- `src/NATS.Server/LeafNodes/LeafConnection.cs`
+- `src/NATS.Server/LeafNodes/LeafNodeManager.cs`
+- `src/NATS.Server/Subscriptions/SubList.cs`
+
+Focus:
+- account-correct remote delivery path (not global-only shortcuts)
+- idempotent remote interest propagation under reconnect/duplicates
+- durable reply-remap and loop-marker transparency rules
+
+### B. MQTT protocol parity
+Primary files:
+- `src/NATS.Server/Mqtt/MqttProtocolParser.cs`
+- `src/NATS.Server/Mqtt/MqttConnection.cs`
+- `src/NATS.Server/Mqtt/MqttListener.cs`
+- `src/NATS.Server/MqttOptions.cs`
+- `src/NATS.Server/Configuration/ConfigProcessor.cs`
+
+Focus:
+- replace line-oriented pseudo protocol with byte-level MQTT packet parsing/writing
+- implement connection/session/subscribe/publish acknowledgement contracts
+- align MQTT auth/TLS/keepalive behavior with configured options
+
+### C. JetStream runtime parity
+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/AckProcessor.cs`
+- `src/NATS.Server/JetStream/Consumers/PullConsumerEngine.cs`
+- `src/NATS.Server/JetStream/Consumers/PushConsumerEngine.cs`
+- `src/NATS.Server/JetStream/MirrorSource/*`
+
+Focus:
+- policy-divergent retention runtime behavior (`Limits/Interest/WorkQueue`)
+- deterministic delivery/ack/backoff/replay/flow-control state-machine transitions
+- mirror/source behavior under filters, transform, and cross-account paths
+
+### D. Storage + RAFT + governance parity
+Primary files:
+- `src/NATS.Server/JetStream/Storage/FileStore.cs`
+- `src/NATS.Server/JetStream/Storage/MemStore.cs`
+- `src/NATS.Server/Raft/RaftNode.cs`
+- `src/NATS.Server/Raft/RaftReplicator.cs`
+- `src/NATS.Server/Raft/RaftTransport.cs`
+- `src/NATS.Server/Raft/RaftSnapshotStore.cs`
+- `src/NATS.Server/JetStream/Cluster/JetStreamMetaGroup.cs`
+- `src/NATS.Server/JetStream/Cluster/StreamReplicaGroup.cs`
+
+Focus:
+- durability invariants across restart/prune/recovery cycles
+- term/quorum/next-index/snapshot/membership semantics with convergence behavior
+- meta/replica governance actions reflected in committed state and routing outcomes
+
+### E. Operational parity surfaces
+Primary files:
+- `src/NATS.Server/Monitoring/MonitorServer.cs`
+- `src/NATS.Server/Monitoring/PprofHandler.cs`
+- `src/NATS.Server/Configuration/ConfigReloader.cs`
+- `src/NATS.Server/NatsOptions.cs`
+- `src/NATS.Server/Events/InternalEventSystem.cs`
+
+Focus:
+- replace synthetic profiling behavior with parity-meaningful runtime profiling outputs
+- close option-surface vs runtime-semantics drift for key advanced options
+- align event/monitor behavior with capability claims
+
+### F. Documentation and evidence
+Primary files:
+- `differences.md`
+- `docs/plans/2026-02-23-jetstream-remaining-parity-map.md`
+- `docs/plans/2026-02-23-jetstream-remaining-parity-verification.md`
+
+Focus:
+- remove contradictory parity claims
+- ensure every claimed closure maps to concrete behavior and test evidence
+
+## 3. Data Flow and Behavioral Contracts
+
+1. Inter-server delivery contract  
+- route/gateway/leaf inbound delivery resolves with correct account scope and permissions  
+- reply-remap and leaf-loop markers remain transport-internal and reversible  
+- remote interest convergence stays idempotent across reconnect churn
+
+2. MQTT protocol contract  
+- fixed-header/remaining-length packet framing must be valid on read/write paths  
+- QoS/session/ack transitions must be deterministic and stateful  
+- auth and account association must fail closed on invalid credentials or malformed packets
+
+3. JetStream stream contract  
+- retention modes diverge by runtime behavior, not only config parsing  
+- preconditions and policy guards execute before irreversible storage changes  
+- max-age/per-subject/dedupe behavior remains consistent under load and restart
+
+4. JetStream consumer contract  
+- deliver policy cursor initialization and progression are deterministic  
+- ack/redelivery/backoff/max-deliver transitions preserve monotonic pending/floor invariants  
+- push/pull flows maintain consistent flow-control and replay timing contracts
+
+5. Storage/RAFT/governance contract  
+- stream/store state invariants remain stable after prune/recovery/snapshot cycles  
+- commit visibility is quorum-gated with stale-term and mismatch handling  
+- governance actions (stepdown/placement/cluster forwarding) reflect committed consensus state
+
+## 4. Error Handling, Testing Strategy, and Completion Gates
+
+### Error handling
+1. Preserve protocol-specific error surfaces across NATS, JetStream, and MQTT.  
+2. Fail closed on account-scoping, authorization, and consensus-precondition violations.  
+3. Prevent partial mutation on multi-step operations (publish+replicate, control+governance).
+
+### Testing strategy
+1. Test-first per capability batch (failing test first, then minimal implementation).  
+2. Each capability requires positive + edge/failure coverage.  
+3. Multi-node, transport, RAFT, and MQTT protocol semantics use integration tests; low-level state transitions use focused unit tests.  
+4. No parity claim is accepted from docs-only or hook-only evidence.
+
+### Completion gates
+1. Capability closes only with Behavior + Tests + Docs complete.  
+2. Full verification gate must pass:
+- focused suites for each completed batch
+- `dotnet test -v minimal`
+3. `differences.md` may only claim closure where behavior is demonstrably ported.  
+4. Deferred parity items are allowed only with explicit blocker rationale and observable evidence.